WRITEUP ON SAFETY VALVE & ERV

28 

Full text

(1)

SAFETY VALVE

1.0 INTRODUCTION

The safety valves are manufactured in India by Bharat Heavy Electricals Limited, Trichirappali under the license agreement with M/s.Dresser Industrial Valve Division,U.S.A. These valves are intended for service in power boilers at pressures upto 213 barg and temperatures upto 593°C. These valves are manufactured, tested and certified as per Indian Boiler Regulations.

The Pressure Relief valves are used in mainly in the following locations: 1. Boiler drums

2. Superheater

3. Reheater inlet (CRH) 4. Reheater outlet (HRH) 5. Soot blower steam line 6. Pressure Reducing Stations

7. Pressure Vessels like Blow down tank.

The critical parameter of a modern high-pressure steam generator is the System Pressure. Though the 'System Pressure' is kept under close control, it can vary beyond limits due to sudden load changes, blocking of outlets, changes in feed water flow, variations in heat input to furnace failure of control instruments and interlocks etc., Under these circumstances the pressure relieving systems in the steam generator play an important role for the safety of operating personnel and capital equipments.

The basic function of a safety valve is to prevent the system pressure in a steam generator exceeding a predetermined value by automatically discharging the steam as soon as the maximum pressure is reached. Besides operating at the set pressure, safety valves must be capable of discharging the full evaporative capacity of the boiler to avoid the possibility of continued pressure build up.

2.0 CLASSIFICATION OF PRESSURE RELIEF VALVES

Pressure Relief valves can be classified into the following. (a) Safety valves

(b) Relief valves

(c) Safety Relief valves

(2)

An automatic pressure-relieving device actuated by the static pressure upstream of the valve characterized by rapid full opening or pop action. It is used for gas or vapour service.

b) RELIEF VALVE

An automatic pressure-relieving device actuated by static pressure upstream of the valve, which open in proportion to the increase in pressure over the opening pressure. It is primarily for liquid service.

c) SAFETY RELIEF VALVE

An automatic pressure-relieving device, which may be used as either a safety or relief valve depending upon application. It is primarily a safety valve with simpler and economical design.

Safety relief valves are usually of the closed bonnet type. To prevent escape of medium to surroundings special Bellow design in safety Relief valves are available for use in back pressure applications.

This discharge capacity had considerable bearing on evolutions of modern high duty safety valves. (Due to many boiler explosions in the earlier stages of development, the national codes for boilers and pressure vessels were formed. The safety valves become essential provisions of National Codes). These codes pay heavy attention towards the selection, design, manufacture, testing and performance of Safety valves on boilers and pressure vessels, as the failure of this equipment may spell failure of the whole plant to which they are adjunct.

3.0 TERMINOLOGY: Accumulation

Accumulation is the pressure increase over the maximum allowable working pressure of the vessel during discharge through the pressure relief valve, expressed as a percentage of set pressure, or actual pressure units.

Back Pressure

Backpressure is the pressure on the discharge side of a pressure relief valve. (Also see “Built-Up Back Pressure” and “Superimposed Back Pressure”, below).

Blowdown

Blowdown is the difference between opening pressure and reseating pressure of a pressure relief valve, expressed as a percentage of the set pressure, or actual pressure units.

Built-Up Back Pressure

Built-up backpressure is pressure, which develops at the valve outlet as a result of flow, after the pressure relief valve has been opened.

Chatter

Chatter is the abnormal, rapid reciprocating motion of the movable parts of a valve in which the disc contacts the seat.

(3)

Closing Pressure

Closing pressure is the point at which the valve re-seats. Closing pressure on a test stand may differ from the blowdown, which is the closing pressure under actual service conditions.

Cold Differential Test Pressure (CDTP)

Cold differential test pressure is the set pressure at which the valve is adjusted to open on the test stand. This pressure includes the corrections for backpressure and/or temperature service conditions

Differential Between Operating and Set Pressures

Valves in process service will generally give best results if the operating pressure does not exceed 90% of the set pressure. However, on pump and compressor discharge lines, the differential required between the operating and set pressures may be greater because of pressure pulsations coming from a reciprocating piston. It is recommended that the valve be set as high above the operating pressure as possible.

Flutter

Flutter is the abnormal, rapid reciprocating motion of the movable parts of a valve in which the disc does not contact the seat.

Lift

Lift is the actual travel of the disc away from the closed position when a valve is relieving. Maximum Allowable Working Pressure

Maximum allowable working pressure is the maximum gauge pressure permissible in a vessel at a designated temperature. A vessel may not be operated above this pressure, or its equivalent, at any metal temperature other than that used in its design. Consequently, for that metal temperature, it is the highest pressure at which the primary safety relief valve is set to open.

Operating Pressure

The operating pressure is the gauge pressure to which the vessel is normally subjected in service.

Overpressure

Overpressure is a pressure increase over the set pressure of the primary relieving device. Overpressure is similar to accumulation when the relieving device is set at the maximum allowable working pressure of the vessel. Normally, overpressure is expressed as a percentage of set pressure.

Rated Capacity

Rated capacity is the percentage of measured flow at an authorized percent overpressure permitted by the applicable code. Rated capacity is generally expressed in kilograms per hour (kg/hr) for vapors; normal cubic meters per minute for gasses; and litres per minute (L/min) for liquids.

Seat Tightness Pressure

Seat tightness pressure is the specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure.

(4)

Set Pressure

Set pressure is the gauge pressure at the valve inlet, for which the safety valve has been adjusted to open under service conditions. In liquid service, set pressure is determined by the inlet pressure at which the valve starts to discharge. In gas or vapor service, the set pressure is determined by the inlet pressure at which the valve pops.

Simmer

Simmer is characterized by the audible passage of a gas or vapor across the seating surfaces just prior to “pop”. The difference between the “start to open pressure” and the set pressure is simmer, and is generally expressed as a percentage of set pressure. Superimposed Back Pressure

Superimposed backpressure is the pressure in the discharge header before the safety relief valve opens. This can be further defined as follows:

Constant Superimposed

This type of backpressure remains essentially at a fixed value (constant) and exists (superimposed) continuously prior to and during opening of the valve.

Variable Superimposed

This type of backpressure varies or changes over a range from a minimum to a maximum, or vice versa. The actual backpressure at any specific time depends on conditions in the piping system to which the outlet of the valve is connected.

Valve Trim

Valve trim includes the nozzle and disc.

4.0 CODES GOVERNING SAFETY VALVES:

To govern boilers and pressure vessels each nation has evolved national and industrial codes. The codes are based on Past experience in the field and provides safety for operation and working of boilers and pressure vessels. The manufacturers and users of these ranges of products must take care of the code rules for the selection, design, manufacture, testing and operation.

The important national codes and industrial standards governing safety valves are: a) IBR

b) ASME Sec.-I, Power Boilers

c) ASME Sec.-VIII, Unfired Pressure vessels d) BS 769

e) Power Test Code PTC 25.3 f) API Stds, 520, 526 & 527

(5)

The factors to be considered in Safety Valve selection and sizing for steam generators for use in India are:

a) The valves must confirm to the Indian Boiler Regulations 1950 with all its amendments,

b) Direct spring loaded valves are preferred

c) The Safety valves used for boilers and pressure vessels are of the full lift type, The orifice area of the safety valve = the peripheral discharge area.

If 'D' is the diameter of orifice and 'L' is the lift of valve, We get πxD²/4 = πDL

Hence the full lift of the valve, L = D/4

d) Minimum two safety valves are to be provided in drum.

e) The drum safety valves must discharge at least 75% of the maximum evaporation of the steam generators.

f) The Super heater safety valves must discharge at least 20% of the maximum evaporation of the steam generator. Correction factor of superheated steam is to be taken into account for capacity calculations.

g) The combined relieving capacity of both drum and superheater valves shall exceed 100% maximum evaporation of the steam generator.

h) Reheater outlet safety valves shall discharge at least 20% of reheater flow and combined reheater inlet and outlet valves must discharge at least total flow through reheater.

i) Generally welded inlet valves are preferred for high-pressure applications.

j) The final selection of safety valves is done to satisfy the capacity and pressure restrictions and to yield the best economy.

k) A power assisted relief valve may be provided on SH outlet to take care of the pressure surges in boiler operation. This valve prevents the frequent lifting of the main spring-loaded safety valves on boiler drum and super heaters due to minor pressure variations. The discharge capacity of the power assisted relief valve is not accounted for the combined relieving capacity of all safety valves.

6.0 RODUCT REQUIREMENTS:

The Boiler Safety Valves should be designed, manufactured and tested to meet the following product characteristics. Tests are to be conducted with saturated steam for acceptance of the product.

a. The valve shall be hydraulically tested to 1.5 times the maximum rated pressure b. The discharge capacity of the valve should be proved by type test.

c. The lift of the valve should be measured and proved by type test.

d. The Reseating pressure (closing down pressure) should be determined experimentally by type test.

e. The computed blow down = (Opening pr. - Reseating pr.)/ Opening pr. x 100%

(6)

f. The operating pressure of the valve at which no leakage occurs should be tested at 94% of set pressure (BHEL Valves).

7.0 CONSTRUCTIONAL FEATURES:

Typical constructional features of a modern heavy-duty safety valve are:

a) Inlet nozzle is profiled to give the maximum discharge.

b) Thermo disc seat provides extremely tight closure and compensates for temperature variation around the periphery of the seat bushing in steam service. Thermal stresses are minimized, seat distortion is prevented and permanent tightness is achieved. c) Backpressures aided closing, which results in positive and precise closing of the valve. d) Lower adjusting ring is used to obtain a clear popping action and to cushion the closing

action of the valve.

e) Upper adjusting ring is used to obtain required blow down in addition with attain full lift at the popping pressure. Its position also determines the point at which the valve begins to drop out of full lift and starts the closing portion of its cycle.

f) Ring pins are provided to keep the ring settings in position.

g) Hydrostatic plug for hydraulic testing of the boilers where the welded valves are site mounted. These plugs are to be replaced by disc after hydraulic test of the boilers. 8.0 OPERATION & MAINTENANCE:

Though the valves are set at factory, the valves after installation should be floated and adjusted to have clear popping action and blow down in the actual operating conditions. The set procedures laid out by manufacturers are to be followed in adjusting the valve. It is not necessary to remove flanged or welded valves from boiler for any maintenance. The actual maintenance required are lapping plate, lapping compounds, high temperature lubricants and ring laps. Spindle run out, which results due to over gagging should be checked and corrected.

BHEL Safety Valves are manufactured as per the technology obtained from M/s Dresser Industries, Inc., U.S.A., The product range covers both safety and safety relief valves. For the power boiler applications, the following pressure relief valve product ranges are manufactured and meets the requirements of IBR.

1. 1700 Series Safety Valves:

This is Dresser's maxiflow design with the product specialties a. Use of thermo disc

b. Two adjusting ring control

(7)

This series of valves are mainly used in Boiler drum, S.H. & R.H. system. 2. 1811 Series Safety Valves

This is a low cost safety valve product line with two-ring control. The valves are designed only up to maximum pressure of 50 kg/cm²g. These valves are used as soot blower line safety valves, continuous blow down and PRDS stations. These valves are now available with improved disc for better performance in high temperature application.

3. 1900 P Series Safety Relief Valves

These valves are of the closed bonnet design. Open yoke designs are also available. Thermo discs are steam application for better leak tightness of valve seats. These valves are selected for pipelines applications for our boiler units in PRDS Systems and CBD tank etc.,

4. 1900 Series Safety Relief Valves

These valves are mainly used in the liquid service and compressible fluid service. Typical application in power plants are Feed water heaters, oil systems and compressed air services.

9.0 SAFETY PRECAUTIONS

• Do not go near discharge side of a safety valve.

• Body drain and cover plate vent must be piped to a safe area. If left open, steam will escape and present a burn hazard to personnel near the valve.

• Always gag a safety valve before making ring adjustments.

• When pulling hand lever for inspection purposes, a rope should be attached to the handle of sufficient length (minimum 6 to 10 m long) to protect the operator from escaping steam, dust, fly ash etc.,

• Exercise caution when examining a safety valve for audible leakage. SUPER HEATED STEAM IS NOT VISIBLE.

• Safety valves should be mounted to provide adequate access, 360° around the valve plus overhead, for disassembly and maintenance.

• Use ear-muffs during the adjustment of valves.

• Use hand gloves during adjustment of valve in the hot condition. 10.0 HANDLING, STORAGE AND PRE -INSTALLATION

Safety valves should be stored in a dry environment to protect them from the weather. They should not be removed from the skid’s or crates until immediately prior to installation. Flange protectors end covers and sealing plugs should also be installed until just prior to installation. When Safety Valves are uncrated and the flange protectors removed immediately prior to installation, meticulous care should be exercised to prevent

(8)

dirt and other foreign materials from entering the inlet and outlet ports while bolting or welding in place. Ensure the disc is available. Identify the disc and store it safely with D.U sequence required for assembly. Check for disc identification dimension before assembly.

The valve, either crated or uncrated, should always be kept with the inlet down i.e., never laid on its side, to prevent misalignment and damage to internals. Keep valve on cradle after removing from crate. Uncrated valves should be moved or hoisted by wrapping a chain or sling around discharge neck, then around upper yoke structure in such a manner that will insure the valve in vertical position during lifting i.e., not lifted in horizontal position. Never lift the full weight of the valve by the lifting lever. Never hook to the spring to lift. Crated valves should always be lifted with the inlet downwards. Safety valves, either crated or uncrated, should never be subjected to sharp impact. This would be most likely to occur by bumping or dropping during loading or unloading from a truck or while moving with a power conveyor, such as a fork lift truck. While hoisting to the installation, care should be exercised to prevent bumping the valve against steel structures and other objects.

10.1 Check list for Storage, Erection and Commissioning

i. Check for tag No.or D.U.No. as per packing slip. Non-availability or discrepancy must be informed to the manufacturing unit. Check for test certificates for each safety valve. ii. Check whether the inlet size, orifice, material grade, set pressure etc. are as per the

boiler O&M Manual.

iii. Check that the seals are intact on ring pin, spring adjuster and overlap collar. Check for transit damages, if any.

iv. Ensure that the end covers are kept in position and removed only just before erecting in place. Meticulous care should be taken to ensure dirt or foreign particles do not enter into the inlet or outlet of the valve.

v. Ensure the valves are stored in crates on sleepers in a covered area with the inlet side downwards. Valves should never be laid on the side to avoid damage to internals. Erection

vi. Ensure that the disc is available, identify and keep in a safe place for final assembly as per D.U. sequence. Identify and hand over the Hyd.test plugs to the user after the Hyd.test is completed.

a. Ensure proper preservation for exposed surfaces of valve and service tools for long storage.

b. Ensure the valves are never subjected to impact by bumping or dropping during loading and unloading and disassembly and assembly of valve.

c. Do not lift valve with lever or spring. Sling valve as described in O&M manual.

vii. Valves should be erected with spindle vertical within a tolerance of ± 1° from the vertical plane.

viii. Ensure proper pre-heating and post heating, stress relieving and NDT procedures are followed when welding to the stub.

ix. Ensure that the vent pipe is separately supported and does not touch the drip pan unit and that there is sufficient allowance for expansion in horizontal and vertical directions.

(9)

x. Ensure the cover plate vent and drain are connected with pipe of adequate size to safe locations.

xi. Extend insulation of vent pipe over roof to avoid rain water collection into drip pan. Insulate the inlet and body upto the cover plate.

xii. After hydrostatic test of boiler, remove hydro test plug and ensure dice is replaced and valve reassembled without affecting setting as per O&M.

Commissioning

xiii. Use calibrated pressure gauges for safety valve floating. Float the lowest set safety valve first, gag the valve after setting is over and float the next higher set valve and so on till all valves are floated and checked for the set pressure within ± 1% tolerance level.

xiv. Try to achieve the setting within a minimum no. of pops (about 3 pops). xv. Check that blowdown is within allowable limit by IBR.

xvi. Apply gag load finger tight in hot condition to avoid spindle bending. xvii. Remove gags after floating of safety valves.

xviii. Check and ensure sufficient approach and maintenance space all around the valve. xix. Proper structures for mounting the slings or pulley blocks are necessary for

maintenance purposes. Same may be checked during installation itself.

xx. After the valves are set in the field, release collar should be taken up and locked with cotter pin such that minimum gap of 3.2 mm (1/8”) is retained between the collar and top lever.

11.0. RECOMMENDED INSTALLATION PRACTICE

i. The safety valve or valves shall be connected be valve or valves shall be connected to the boiler independent of any other connection, and attached as close as possible to the boiler, without any unnecessary intervening valve or fitting. Such intervening pipe or fitting shall not be longer than the face-to-face dimension of the corresponding tee fitting of same diameter and pressure. Every safety valve shall be connected so as to stand in an upright position with spindle vertical.

ii. The opening or connection between the boiler and the safety valve shall have at least the area of the valve inlet. No valve of any description shall be placed between the required safety valve and the boiler nor on the discharge pipe between the safety valve and the atmosphere. When a discharge pipe is used, the cross-sectional area shall not be less than the full area of the valve outlet of the total of the areas of the valve outlets discharging there into. It shall be as short and straight as possible and so arranged as to avoid undue stresses on the valve or valves. All safety valve discharges shall be so located or piped as to be carried clear from running boards or platforms. Ample provision for gravity drain shall be made in the discharge pipe or near each safety valve or safety relief valve, and where water or condensation may collect. Each valve shall have an open gravity drain through the casing below the level of the valve seat. For iron and steel-bodied valves exceeding 63.5 mm (2-1/2") size, the drain hole shall be tapped not less than 9.5 mm (3/8") pipe size.

(10)

iii. If a muffler is used on a safety valve or safety relief valve, it shall have sufficient outlet area to prevent back pressure from interfering with the proper operation and discharge capacity of the valve. The muffler plates or other devices shall be so constructed as to avoid a possibility of restriction of the steam passages due to deposits. When a safety valve is exposed to outdoor elements which may affect operation of the valve, it is permissible to shield the valve with a satisfactory cover. The shield or cover, shall be properly vented and arranged to permit servicing and normal operation of the valve. iv. Safety valves may be attached to drums or headers by welding provided the welding is

done in accordance with Code requirements. 11.1 Outdoor Safety Valve Installation

Safety Valves operating under the best possible conditions of favorable operating gap, relatively stable ambient temperatures, absence of dirt and in relatively still air, will provide the maximum degree of safety, tightness and dependability. When a safety valve is installed in a location such as out doors, it is exposed to wind, rain, snow, ice, dirt and varying temperatures.The following recommendations are made for proper protection to insure that operational dependability can be restored to a level near that of the valve installed under ideal conditions. The inlet neck of the safety valve and safety valve body, upto the bottom of the cover plate. Should be insulated. The exterior surface of any such insulation should be weather proofed by any suitable means. In addition to maintaining a more even temperature within the valve body, during widely fluctuating ambient temperatures, this insulation will effectively reduce thermal stresses due to high temperature gradients through the walls of the safety valve nozzles. Even in tropical latitudes, insulation should be applied since safety valves installed in outdoor locations in these areas are subject to hurricane conditions. Spring covers should be used to stabilize as nearly as possible the temperature of the spring and also to prevent the accumulation of dirt, ash, and ice between coils of the spring. Lifting gear covers should be installed to prevent ice, dirt and fly ash from accumulating in areas inside the safety valve cap.

11.2 Field Setting

All Safety valves are steam tested at the factory. Every valve is set to have a clean popping action and to reseat tightly. However, because the boiler used in setting the valves has a small capacity compared to the capacities of the valve. Slight adjustments on the actual installation are necessary to maintain proper action and blowdown. Two methods exist for field testing of valves ;use of the 1566 or 1566-2, Hydroset Unit, and full system pressure.

As previously noted, the use of a hydroset unit will serve to establish Set pressure only and should not be used for establish blowdown, lift, etc., Gagging of other valve not being set generally will not be necessary; however, for setting of high set pressure valves, depending on system pressure being used, it may be required to gag the lower set valves. Boiler safety valve tests can be conducted with the unit either on or off the line. However, with the unit on the line at full load a sudden load drop could be dangerous as most of the safety valves will be gagged. Therefore, it is recommended that the safety will be tested and adjusted with unit off the line or with light load. Boiler control can then be

(11)

maintained with little or no outside influence due to load change. The valves should be set within ±1% of the set pressure as per ASME Sec-1. Set pressure should not be changed without the permission of the manufacturing unit. Lab standard or test Gauges should be used with a minimum graduation of 0.5% of the full scale reading and accuracy of ±0.25%.Gauges should be of sufficient range so that reading is in the middle third of the scale. Factors Which Can affect Safety valve operation are:

i. Ambient temperature

ii. Discharge Stack binding against drip pan elbow. iii. Vibration of the header from upstream bends, etc. iv. Operating gap.

12.0 HYDROSTATIC TESTING AND GAGGING

12.1 During any hydrostatic test all safety valves on the unit must be gagged. This gagging procedure prevents the possibility of damage of the safety valve internals in the event that the test procedure exceeds the safety valve set pressure. When valves are subjected to working hydrostatic tests not exceeding the set pressure of the low set valve, valves may be gagged rather than using hydrostatic test plugs. For higher pressures hydrostatic plugs should be used and valves should be gagged. Probably the most common source of Safety Valve trouble is over gagging.

During Hydrostatic Testing and Safety Valve Setting Gags should be applied only hand tight. During setting, over gagging will also cause damage to the seating surface and the resultant leakage. In applying gags remember that the valve spring will hold the valve closed against its set pressure. The additional gag load applied should be only enough to insure that the valves do not lift at the expected over-pressure.

Gags should never be applied when the boiler is cold. The spindle of the safety valve expands considerably with the temperature increase as pressure is raised. If it is not free to expand with this temperature change it may become seriously bent. Boiler pressure should be brought up to within 80% of the pressure of the low set valve before applying the gags. Tighten the gags of drum and superheater valves with only a light force applied to the gag stem. Reheater valves require a greater force applied to the gag stem to hold the valve closed during hydrostatic test.

Application of Test Gags (All Pressures)

i. Remove top lever pin (130), top lever (128) then loosen cap screw (238) remove cap (126) and Drop lever (127) as an assembly. The release collar (131) is fixed to spindle (046) by means of a cotter pin (185). Note that the release collar (131) does not quite engage top of spring adjuster (101). See fig for details or release collar position.

ii. Center the test gag stem on the exposed end of the spindle (046) and hook the legs of gag under the sides of the yoke.

DO NOT APPLY THE GAG LAOD UNTIL THE BOILER HYDROSTATIC PRESSURE IS EQUAL TO 80%OF THE PRESSURE TO WHICH THE LOW SET VALVE IS ADJUSTED.

iii. Apply the gag load by turning the gag stem clockwise. Any attempt to pinch off the leakage through the valve without first lowering the hydrostatic pressure is liable to result in damage to the valve seats. If the gag on the valve has not been tightened sufficiently, the valve will leak and sometimes the leakage is accompanied by “Sizzling”

(12)

sound. If this occurs the hydrostatic test pressure should be reduced until the valve becomes tight and then gag should be tightened further. This procedure must be followed exactly since it is very difficult to stop the leak by additional gagging once it has started.

iv. After the hydrostatic test is completed, the gags should be removed when the hydrostatic pressure has been reduced to 85% to 90% of the pressure of the low set valve.

Nomenclature List:

002 Base

(13)

004 Inlet Neck 008 Yoke 009 Yoke rod 011 spring

012 Upper Spring Washer 016 Lower Spring Washer 026 Disc

028 Disc Holder 029 Disc Collar 031 Guide

036 Upper Adj.Ring 037 Upper Ring Pin 038 Lower Adj.Pin 039 Lower Ring Pin 046 Spindle

052 Lift Stop 054 Overlap Collar 056 Cover plate

057 Lower Washer Retainer 058 Lower Floating Washer 059 Lower washer Retaining Pin 061 Top Plate

062 Upper Washer retainer 063 Upper Floating Washer 064 Upper Washer Retaining Pin 101 Spring Adjuster

102 spring Adjuster Lock Nut 105 Service Port Plug

126 Cap

127 Drop Lever 128 Top Lever 129 Drop Lever Pin 130 Top Lever Pin 131 Release Collar

182 Disc Collar Cotter Pin 183 Lift Stop Cotter Pin 184 Overlap Collar Cotter Pin 185 Release Collar Cotter Pin 226 cover plate stud

229 cover plate nut 231 Yoke Rod Nuts

232 Lower Yoke Rod Nuts 237 Retainer screw (Top Plate)

(14)

238 Cap Set Screw 13.0 Factory Setting Vs. Field Setting

Every Safely Valve is set and adjusted on steam before shipment from the factory. Blowdown adjustments are made is carefully and accurately as possible on the factory test boiler. However it must be recognized that actual field operating conditions may vary considerably from factory test conditions.

Conditions beyond the manufacture’s control that affect safety valve operation are:

i. Quantity of steam being discharged through the valve, i.e. the actual installation capacity exceeding that of the test boiler, thus permitting the valve to flow its full rated capacity.

ii. Quality of steam being discharged.

iii. Discharge piping stresses and back pressure. iv. Ambient temperature.

v. Shipping or storage damage. vi. Improper gagging.

vii. Improper bolting of flanges

viii. Damage due to foreign material in the steam.

xi. Damage, during disassembly and assembly in the field xii. Insufficient operating gap

xiii. Vibrations

Final Safety Valve adjustments made on the actual installation is the best means of ensuring that the valves perform in compliance with the applicable code requirements. 13.1 Popping Point Adjustment

To change the popping pressure of the valve remove the cap and lever assembly, loosen the locknut and turn the spring adjuster clockwise to increase pressure. After each adjustment of the spring adjuster the locknut should be tightened. The arm on top of spring washer should always be free from bearing against the yoke rod. This can be accomplished by holding a screwdriver between the arm and the rod to prevent any movement of the top spring washer while adjusting the spring adjuster.

13.2 Ring Adjustment:

Always gag the Safety Valve for protection, in case boiler pressure rises while making ring adjustments (See instructions for gagging). The positions of the upper adjusting ring and the lower adjusting ring are locked by means of the upper adjusting ring pin and the lower ring pin respectively. These pins are threaded into the valve body and engage notches, which are cut into the rings. To adjust either ring the corresponding ring pin must be removed. A screwdriver can be used to turn the rings.

Spare ring pins when supplied will be of excess length and have to be individually ground to match each valve. The tip of the pin should not touch the bottom of the ring notches. Lower Ring Adjustment

(15)

Using Figure 1 and Table 1, a reference plane is established where the adjusting ring is level with the bushing seat. From this setting, factory adjustments are made by adjusting one notch downward for each 42.2 kg/sq.cm (600 psi) of set pressure. Ring position should be checked prior to testing. Slight deviations from these settings are possible because of tests conducted in the plant.

NOTE (THIS PROVIDES A STARTING POINT ONLY). The lower ring is used to obtain a clean popping action and to cushion the closing action of the valve.

DO NOT ATTEMPT TO ADJUST BLOWDOWN WITH THE LOWER RING.

The ideal position must then be found for the set of operating conditions present. If simmer is present or valve fails to lift, the lower ring should be moved upward slowly, one notch at a time to remove the simmer. The most ideal position for the lower ring is the lowest position that does not introduce simmer or a buzzing sound. In this connection, it is imperative that extreme care be used in conditioning the seat surfaces, ensuring correct alignment, and establishing the proper clearance, so that mechanical causes of simmer will be reduced to a minimum.

Upper Ring Adjustment

When shipped from the factory, upper adjusting rings are set level with the bushing seat in accordance with Figure 1 and Table 1. This should be checked prior to field setting and should be regarded as a starting position only. It is possible that this position is not strictly in accordance with Figure 1 and Table 1 on some valves because of testing at the plant. The ring may be set lower than the seat level, but never before, until field tests are conducted. Starting position for blowdown check should place bottom surface of upper ring level with bushing seat. Since the bushing seat is not visible through the service ports. It is necessary to use a cross-reference relationship. Raise the upper ring until its bottom surface in level with the disc holder, which is visible through the ports. Then, referring to Table 1, Column B, lower the upper ring the required number of notches. This will place the upper ring on a plane with the bushing seat. This is the starting point for blowdown tests. Further adjustments may be necessary depending on test results. When further adjustments are required to obtain final blowdown setting, the upper adjusting ring should be moved 5-10 notches at a time as follows:

(16)

TABLE – 1

ADJUSTING RINGS FINAL FACTORY POSITIONS (FIELD SETTING STARTING POSITIONS)

Orifice Lower Ring Holder to Seat in notches (Column – A) Upper Ring Holder to Seat in notches (Column – B) 1 7 10 2 8 12 3 12 16 4 12 16 5 12 16 6 30 45 7 30 45 Q 30 45 8 37 45 R 38 47

(17)

Gag the safety valve for protection while making ring adjustments. The lower and upper adjusting ring pins of any particular valve are not interchangeable. Spare ring pins when supplied will be of excess length. These pins are to be ground at site so as to match the valve under maintenance. The tip of pin should NOT touch the bottom of the ring notch (refer sketch enclosed) Also the pins should not bear against the rings. DO NOT attempt to adjust blow down with the lower ring. The upper adjusting ring should not be set below the seat level and the lower adjusting ring should not be set above the seat level. Lock both the ring pin settings using a seal wire to avoid disturbance during system/valve operation and line vibrations.

RECOMMENDED

NOT RECOMMENDED

NOT RECOMMENDED

BODY ADJUSTING RING

ADJUSTING RING PIN

BODY

BODY ADJUSTING RING PIN ADJUSTING RING

ADJUSTING RING ADJUSTING RING PIN

(18)

Ensure that the above settings are properly locked: Periodically check whether the pins are in their appropriate place and are effective. This can be checked through the service port, provided in the valve body. A sketch showing acceptable ring pin positions is enclosed. Excessive line vibrations, presence of foreign particles, discharge piping stress, back pressure, reaction force etc., can lead to pin failure over a prolonged period of exposure. The ring pins being the most critical spare part, it is desirable that one pin per valve (for drum and SH SVs) and one ring per four valves (for RH SVs) is stocked as essential spares by customer. Adherence to the above aspects would eliminate cases of premature pin failure and lead to a satisfactory valve performance.

To reduce blowdown -MOVE RING UP-TURN COUNTER CLOCKWISE. To increase blowdown -MOVE RING DOWN-TURN CLOCKWISE.

It is possible to raise the upper ring too far and prohibit attainment of full lift. When this occurs, lower to the point where full lift is attainable and finalize the blowdown setting with the overlap collar (See Overlap Collar Position). If valve fails to lift the lower adjusting ring requires further adjustment. (See Lower Adjusting Ring Position)

Check the ring pins to see that the engage the ring grooves, but without touching the bottom. The pins should not bear against the rings.

GUIDELINES FOR THE STEAM TESTING OF BOILER SAFETY VALVES

Implementing the following procedures will benefit your company in good valve performance, reduced labor time and fuel savings.

Mechanical

1. We highly recommend a preliminary test at reduced pressures of the boiler valves with our 1566 hydroset prior to steam. Savings in fuel and time will easily justify this step. 2. Only calibrated test gauges should be used for testing safety valves. Gauges not

calibrated can cause problems with operating pressure later or even necessitate the retesting of all the safety valves.

3. Gage for each valve should be available and in good condition.

4. Discharge piping has to be inspected for binding on the valves, supports and welds on piping. Binding of the discharge could make the valve hang up on closing. Insufficient welds and supports could cause the discharge piping to fly off during valve activation. 5. A rope approximately 5 to 7 meters with a hook one end should be attached to the

valve lifting lever before activation. Should the valve chatter or not lift at set pressures, the rope can pulled, Eliminating chatter and stop over pressure of the vessel.

6. Have the correct tooling available. Looking for tools means lost lime. Having incorrect tools can cause damage to compression screws and other valve parts.

7. Establish a good communication system between the boiler and control room. This eliminates mistakes that could be dangerous and time consuming.

Operational

1. If the unit has a power assisted relief valve “Electromatic”, place this valve in operation first. Since safety valves are tested one at a time and the other valves are gagged, the

(19)

pilot actuated relief valve will increase the safety factor of the unit while testing. Pilot operated valves can be actuated from the control room if some emergency should develop during the controlled burner firing associated when tests are being conducted. The electromatic will help clean contamination from system.

2. The Drum valve should be tested first Possibilities of valve part damage because of “girl blasting” are greater on superheated valves in contrast to the drum valves. If a super heated valve is gagged after seat damage while testing of other valves continues, the total valve damage will most likely be increased. Temperatures of the boiler increase during the testing cycle of the Drum Valves. Consequently higher temperature steam will be available for testing the Superheater valves and produce more accurate results.

3. Keep water level down as low as possible while testing drum valves. If drum valves level is high the Safety Valve may be stugged with water causing a long blowdown and possibly damage the valves

4. Pressure should be brought up evenly and fairly rapid rate A slow raise in pressure will increase the simmer of the valve possibly causing the valve to life erratically. We recommended 2 to 3 kg/cm2 per minute increase in pressure during testing to be optimum. Should a burner or other systems fail when pressure is near the set pressure of the valve, drop boiler pressure at least 10 percent below set pressure before raising boiler pressure again.

5. Holding boiler pressure near the set pressure of the valve will induce simmer, possibly causing the valve to lift erratically.

6. Drop pressure after a valve has lifted to a safe level before taking the gag off the next valve or making-valve adjustments. 20 percent of the valve set pressure will assure the non-gagged valve will not lift prematurely.

7. If a valve has to be lifted several times, a cooling off period will be necessary. For outside or inside valve installation, 25-30 minutes is sufficient: If this cooling period is not maintained, erratic operation of the valves can be expected.

8. If the valves have not been tested with the hydroset prior to the steam actuation, it is recommended to hand lift the valves before the actual steam pop. Hand lifting the valves will show that the valves are functional furthermore; it will help to warm up the valve.

Safety

For the protection of persons in testing be valves, a few simple steps can, prevent accidents and injury.

1. Have only people on the boiler absolutely necessary for the testing of the valves. 2. Rope off the area where valves are going to be tested.

3. When working on the valves, wear gloves

4. All personnel on the boiler are to wear eye and ear protection. 5. Warn people on the plant site that valve testing is being performed.

The recommendations we have made represent many years of field experience. We hope you can implement some or all of them. Hopefully, you will have a good start up.

(20)

14.0 SITE PROBLEMS IN SAFETY VALVES:

The general problems faced at various sites for the safety valves are: I. Safety Valve Leakage:

Causes

1. Seat / disc damage 2. Long simmer

3. Physical obstruction in seat. 4. Spindle is bent

5. Wrong working pressure closer than 6% of set pressure. 6. Retaining nut position wrong.

7. High ambient temperature

8. Exhaust pipe expansion clearances not sufficient 9. Impurity in steam/Foreign particles damages. Remedy

1. Change/lap the disc/seat 2. Adjust the lower ring position 3. Change/rectify the spindle 4. Increase the set pr.

5. Change the position so that a clear gap is available.

6. Ambient temperature is to be reduced by identifying source. 7. Change the drip pan or exhaust pipe to get ample clearances

II. Seat damages in both disc and body seats resulting in Seat passing due to foreign particles in steam

Remedy

Relapping of Disc and body seats.

If the damages are heavy, in body seats, due to wire drawing and seat cutting action the roto lap kit can be used for body seats for seat reconditioning. However, final seat lapping is done with ring laps. If the damages are minor hand lapping will be sufficient. If the damages are beyond relapping level the seat is remachined using a reseating machine and subsequently body seat is lapped. The lapping is checked with carbon blue. Attend to safety valve leakage at the earliest otherwise the steam cutting action will damage valve seats and trims requiring more replacement parts.

III. Simmering of Safety Valves

1. Check misalignment of safety valve erection to vertical plane for ±1°. 2. Check vibration of line on which safety valve is mounted

3. Check escape pipe is freely supported and do not transfer any load to safety valve 4. Check minimum operating gap between set pressure and operating pressure is 6%

with ±1% tolerance for set pressure of safety valve. If the operating gap is critical set safety valves to +1% tolerance level.

(21)

5. Raise Lower adj. Ring one notch at a time from the original position as per O&M to overcome simmering of valve.

6. Check for leakage of safety valve. Leaking Valves may cause simmer and erratic behavior of Valve. Attend to Safety valve leakage by lapping valve seats.

7. Check failure of ring pins. This will result in ring position changes leading to simmering of safety valve. Readjust rings as per O&M. Replace broken ring pins, ensuring adequate play between adjusting rings and ring pins.

Do not replace ring pins with manufactured pieces at user works as special materials are used for ring pins.

Caution:

i) While assembling the ring pins, make sure it is not tight against the adjusting rings. Tight pins will lead to breakage.

ii). Lower and upper ring pins are not interchangeable IV. Chattering of Safety Valve:

Cause:

1. Long inlet pipes 2. Inlet restrictions

3. Too short a blow down 4. Too large valve capacity Remedy:

1. Use short stubs for safety valve installation.

2. Inlet opening shall be corrected to a size equal or more than safety valve inlet. Adjust the blow down by blow down adjusting rings. Change the valve to smaller size or restrict the lift.

V. Fly ash entering Safety Valves

This is noted at few sites where the fly ash coming out through the furnace roof due to poor roof sealing and positive pressures in furnaces. These accumulated ashes fall into the trim components of Safety Valves through the cover plate clearances. This leads to scale formation at guide and disc holder and disc spindle thread joints and results in jamming of these components. These may result in Safety valve not fully lifting due to deposits between guide and disc holder.

The valves should be protected with covers in such sites where fly ash is coming out of the boiler through the roof.

6. Spindle bending: Cause: Cold gagging Remedy:

(22)

Replace bent spindle. Always apply gag load on SV spindle when the valve is in hot condition only.

7. Locating safety valves

Service platforms and accessibility for servicing of Safety Valves maintenance are to be given attention at site during erection. A cross beam is required in the centerline of each safety valve for assembly and disassembly of valve.

(23)

ELECTROMATIC RELIEF VALVE

1. GENERAL DESCRIPTION

The BHEL ELECTROMATIC RELIEF VALVE is an electrically actuated pressure relief device. It may be operated, manually at will by closing a switch or automatically by means of a pressure sensitive element to relieve pressure accurately within very close limits. The application of this valve places at the command, of the plant operator a means of instantaneously opening and closing a relief valve on some remote header. The ELECTROMATIC RELIEF VALVE may be used to purge a superheater or a header, thereby decreasing -the possibility of damaging the spring loaded safety valve seats or turbine equipment. With the pressure element set to open the ELECTROMATIC RELIEF VALVE automatically at a pressure slightly below the lowest set spring loaded safety valve, it will effectively prevent the safety valves from lifting except on major over-pressures. Figure - 1 illustrates diagrammatically the relationship of the various elements of the ELECTROMATIC RELIEF VALVE system. The Type 1537 control station is equipped with a three-position (Manual, Off, and Automatic) switch and with three indicating pilot, lights (red, blue and green). The type 2537 VX control station is equipped with two indicating lamps (red and amber). The type 1539 VX controller consists of a pressure sensitive element consisting of a Bourdon Tube which actuates electrical contacts, and a heavy duty relay to switch the solenoid load. The solenoid assembly then operates a pilot valve. The pilot valve in, turn controls the opening and closing of the main valve. Controllers are generally furnished for 220V DC; a resistor unit is built into the solenoid assembly for the purpose of reducing the current when the solenoid is in its holding position. In the case of AC solenoid the resistor unit is not necessary.

2.0 SAFETY NOTICE

Proper service and repair is important to the safe, reliable operation of all valve products. The service procedures recommended by BHEL and described in the maintenance and service manual are effective methods of performing the required maintenance operations. Some of these service operations require the use of tools specifically designed for the purpose. These special tools should be used when and as recommended.

It is important to note that this service manual contain various warnings and cautions which should be carefully read in order to minimize the risk of personal injury or the possibility that improper service methods will be followed which may damage the valve or render it unsafe.

It is also important to understand that these warnings and cautions are not exhaustive. BHEL could not possibly know, evaluate, and advice the customer or utility of all conceivable ways in which service might be done, or of the possible hazardous consequences of each way. Consequently BHEL has not undertaken and such broad evaluation. Accordingly, anyone who uses a service procedure or tool which is not recommended by BHEL must satisfy himself thoroughly that neither his or other personnel’s safety nor valve safety and proper operation will be jeopardized by the service method he selects. Contact BHEL if there is any question on the method.

(24)

The testing, installation and removal of valve and valve products may involve the use of fluids at extremely high pressure and temperature and/or corrosive or erosive. Consequently, every precaution should be taken to prevent injury to personnel during the performance of any test, installation or removal such as, but not limited to, ear drum protection, eye protection, and protective clothing such as gloves, etc., in and around the testing, installation, or removal area. Due to the various circumstances and conditions in which these operations may be performed on our products or the possible hazardous consequences of each way BHEL could not possibly evaluate all conditions that could injure personnel or equipment, but does offer these safety precautions as an assistance only.

3.0 SAFETY PRECAUTIONS

1. Do not stand in front of the discharge side of a pressure relief valve when testing or operating.

2. Hearing protection should be used when testing or operating valve.

3. Exercise caution when examining a pressure relief valve for visible leakage. 4. Never install a pressure relief valve in a horizontal position. Pressure relief valve

internals are designed to move vertically, when installed horizontally, misalignment and galling or hang-up may prevent the valve from opening or closing properly.

5. The body drain must be piped to a safe area. If left plugged, condensate will accumulate inside the valve body. If left open, hot and/or corrosive media will escape and present a hazard to personnel near the valve.

6. Pressure relief valves should be mounted to provide adequate access, 360O around the valve plus overhead to permit removal for testing and maintenance. 7. When removing the pressure relief valve during disassembly, stand clear and/or

wear protective clothing to prevent exposure to splatter of any corrosive process medium which may have been trapped inside. Ensure valve is isolated from system pressure before valve is removed.

4.0 STORAGE AND HANDLING

i. The valve, either crated or uncrated, should always be kept with the inlet flange down, i.e. never laid on its side, to prevent damage to internals of main valve and pilot valve.

ii. Electromatic Relief Valves should be stored in the original shipping crates in a dry environment to protect them from the weather. They should not be removed from the crates until immediately prior to installation.

iii. End covers and sealing plugs i.e. both inlet and outlet should not be removed until the valve is ready to be installed on the system.

iv. Electromatic Relief Valves, either crated or uncrated, should never be subjected to sharp impact. This would be most likely to occur by bumping or dropping during loading or unloading from a truck or while moving with a power conveyor, such as a fork lift truck. While hoisting to the installation care should be exercised to, prevent bumping the valve against steel structures and other objects.

(25)

located on the valve body near the discharge collar

vi. When Electromatic Relief Valves are uncrated and the end covers removed immediately prior to installation, meticulous care should be exercised to prevent dirt and other foreign materials from entering the inlet and outlet ports while installing in place.

(26)

5.0 DO’S AND DON’TS DO’S i. Check the valves for damage.

ii. Weld the valve assembly vertically within ± 1° .

iii. Mount the controller rigidly and away from location of vibrations as indicated in the section ‘CONTROLLER INSTALLATION’.

iv. Slope down the impulse line towards the controller and provide a syphon. V. Check the power supply for correctness with respect to solenoid requirements. vi. Check the cut off switch in the solenoid for proper function.

vii. Check the pressure switch of controller for correct range.

viii. Check the connection and availability of M.O.V. in the controller.

ix. Set or check the reseating pressure at the controller using a hydraulic hand pump and ensure the pilot light function before setting the valve.

x. Check the smooth action of the solenoid by pressing the solenoid. DON’TS

i. Do not exceed the voltage drop at the solenoid (not at source) by more than that indicated in section 8 “ELECTRICAL SUPPLY’.

ii. Do not operate the Electromatic Relief Valve with the pilot valve spindle pressed down by operating lever.

(27)

in the circuit for solenoid with DC power supply. iv. Do not subject the solenoid controller to over heating.

V. Do not connect the impulse line without blowing the line clean.

vi. Do not attempt to set the blow down to 1% without checking the blow down setting limits.

6.0 PRE STARTUP CHECKS

A. FOR VALVES WITH 1537 VX CONTROL STATION AND 1539 VX CONTROLLER i. Close the Isolating gate valve which is below the Electromatic Relief Valve.

ii. Hold the pressure below the popping point and turn the control switch to ‘Automatic’. Now the green light must go on, this indicates that the electrical system is alive

iii. Turn switch to ‘manual’ and observe that the solenoid plunger goes to bottom of stroke and depresses the pilot stem by 2.4 min (3/32") to 3.2 mm (1/8") max.

The Red and Blue lights should now be on. The blue light indicates that the relay is energized.

7.0 FACTORY SETTING VS FIELD SETTING

Every BHEL Safety Valve is set and adjusted on steam before shipment from the factory. Blowdown adjustments are made as carefully and accurately as possible on the factory test boiler. However, it must be recognized that actual field operating conditions may vary considerably from factory test condition. Final safety valve adjustments made on the actual installation is the best means of ensuring that the valve performs in conformance to IBR, ASME and other applicable code requirements.

8.0 SITE PROBLEMS IN ELECTROMATIC RELIEF VALVES: The general problems faced at various sites for the safety valves are: I. ERV-Dismantling of main Valve seat bushing:

1) Use persuader wrench, with 3 feet long pipe. Apply torque in the anti clock wise direction; at the same time hit the opposite end of the wrench with the sledgehammer, so that the seal contact is broken. Apply penetrating oil.

2) If there is a drag or siering and bushing is difficult to turn, do not force the bushing. Apply more penetrating oil.

II. ERV: Pilot Valve Leakage: Causes:

1) Pilot valve seat damage

2) Pilot valve retaining spring broken or became solid 3) Pilot valve seat gasket damaged.

Remedy:

1. Seat to be lapped/changed 2. Change the parts.

(28)

III. Solenoid not working/burnt out Causes:

1. Incorrect voltage

2. Correct voltage but varying beyond tolerable 10% at the solenoid connection. 3. Too high an ambient temperature

4. Ash or debris collections on solenoid Remedy:

1. Change the voltage or install new solenoid corresponding to available voltage 2. Rectify the variation.

3. Identify the source and reduce the ambient temperature or replace the solenoid with new. Insulate if the same is sufficient.

4. Clean and service the parts. Spares and Service tools:

The recommended spare parts and service tools relapping kits, Reseating machine are to be procured and stocked at every site for reconditioning of Safety valves. In the absence of spare parts and service tools at site the servicing of Safety valves becomes difficult. Inch spanner sets are required for Safety valves. Use Master lapping plate for reconditioning ring laps used for Safety Valves.

Service tools and spares for ERV:

The persuader wrench, the lapping tools for main and pilot valves, and the recommended spare parts are to be stocked at site for servicing of ERV.

Dismantle Safety Valves without affecting the settings, Lap seats as per O&M, Measure settings if component wise dismantling is required at site.

1) Spring setting 2) Lift setting

3) Upper Adj.Ring setting 4) Lower Adj.Ring setting 5) Over lap setting

Figure

Updating...

References

Updating...

Related subjects :