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Pressure Relief Valve Piping Failures and Fire: Ammonia Synthesis Loop

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Pressure Relief Valve Piping Failures and Fire: Ammonia Synthesis Loop

This article describes a piping failure and fire resulting from inadequate support of an ammonia synthesis loop pressure relief valve, the nondestructive testing and evaluation of fire-impinged equip-

ment, and the pipe failure mode. It also addresses support criteria f or relief valves and other piping systems illustrating the importance of evaluating both static and dynamic loads.

Lester E. Sutherland and Michael Holman CF Industries, Inc., Donaldsonville, LA 70346

Don Hansen Piping Analysis Inc.

Introduction

O

n May 1, 1996 at approximately 4:15 PM, while in the process of restarting CPUs Ammonia Plant 3 following a nonscheduled maintenance shutdown, a fire occurred in the vicinity of the Ammonia Refrigerant Flash Drums. The cause of this fire was later identified as an insufficiency in the piping/support system for the Synthesis Gas Loop Relief Valve (RV-105D).

CPU's Anmonia Plant 3 is a M. W. Kellogg designed 1,000 TPD Stretch unit that was commissioned in 1976. The First, Second and Third Stage Ammonia Refrigerant Flash Drums (112F, 11 IF, and 11OF, respectively) appear to be a single horizontal drum mounted approximately 20 ft above grade. This drum is separated into 3 distinct chambers by 2 internal heads. The vessel has an inside diameter of 7 ft and is approximately 70 ft in length. The material of con- struction of the flash drum is SA-516 Grade 60 and has a nominal 0.4 in. shell thickness and a 1/16 in. cor- rosion allowance. The vessel was post weld heat treat-

ed after fabrication. The design pressure of the vessel is 100 psig. The 110F operates at 85 psig and 56°F with the 11 IF and 112F operating at progressively lower pressures and temperatures. The drums contain ammonia liquid and vapor and have been in service for approximately 20 years. The drums were insulated during initial plant construction with foam glass insu- lation and aluminum weather proofing. A zinc rich epoxy coating was applied to the shell prior to installing the insulation.

A common platform runs the length of the drum which provides access to an array of level, pressure, and temperature instrumentation. Several relief valves are also serviced from this platform.

RV 105D is a pilot operated relief valve that protects the Synthesis Gas Loop from excessive pressure. The 3 in diameter relief valve inlet is connected to a 14 in.

diameter line through approximately 15 ft of 3 in.

diameter pipe. The pilots, which serve to actuate the relief valve, are connected to the 14 in. diameter line through a separate 1/2 in. diameter remote sensing line. Access to this valve is provided by the above

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mentioned platform.

While restarting the plant, the RV-105D lifted. It was reported by operations personnel that the tail pipe began to vibrate violently and that the valve was chat- tering. Apparently, the stress levels in the 90° elbow immediately below the valve exceeded their allow- ables and the elbow failed. The failure of this elbow allowed the relief valve and tail pipe assembly to lay over into the horizontal plane pointing directly parallel with the axis of the 11OF, 11 IF, 112F platform (see Figure 1). Cracks developed on either side of the elbow and the leaking high pressure synthesis gas eventually ignited. As the valve had still not seated properly, synthesis gas continued leaking from the tail pipe. This gas stream eventually ignited. At this point, fire was emanating from the cracks in the 90° elbow as well as from the tailpipe. The fire continued to bum for approximately l h as the synthesis gas section depressured.

The shock waves, which followed the two ignitions, damaged a great deal of insulation in the general vicinity. The synthesis loop eventually depressured sufficiently to allow the RV to seat.

This incident appears to be the result of at least the following factors:

The Remote Sensing Line was not Properly Connected to the Relief Valve Pilots. During a routine plant turnaround, RV-105D was removed from the unit for inspection and testing. In order to test the valve, the pilots were temporarily connected to the valve inlet. These connections were not removed from the valve prior to its return to the plant. The technicians that reinstalled the valve in the plant did not reconnect the remote sensing line to the pilots, leaving the test lines in place.

With the pilot sensing line connected to the pressure tap located just below the valve seat, problems devel- oped. As the valve was in the flowing condition, a pressure drop in the line occurred. The pressure imme- diately upstream of the valve was instantaneously reduced, allowing the valve to seat. The pressure immediately rebuilt to the header pressure and caused the valve to reopen. This situation was repeated many times per second. The cumulative result was a cyclical load being applied to the support system.

The Tail Pipe was not Properly Cut at the

Discharge. This situation resulted hi the application of a tremendous moment arm and hence extremely high stress levels to the piping system.

The Support System for the RVwas not of Sufficient Strength. The tail pipe was restrained by an angle steel bracket welded to a handrail. This support, more likely than not, failed the instant the valve lifted.

Discussion of Subsequent Inspections

The following is a summary of the equipment inspections performed following the incident. The intention of these inspections was to assess the extent of equipment damage which may have occurred as a result of the fire and to assist in the adoption of intelli- gent remedial measures.

The types of inspections performed on the various pieces of equipment, as well as the acceptance/rejec- tion criteria, were developed by CFÏÏ and were based upon input from several outside engineering firms.

110F

The south side of the west head of the 11 OF was directly exposed to heat from the fire emanating from cracks in the 90° elbow located immediately below the 105D RV. Insulation was absent from this area, appar- ently the victim of one of the shock waves which fol- lowed the ignition of the leaking synthesis gas. AH of the instrumentation and associated small bore piping hi the immediate area were also found to be without insulation and to have been exposed to intense heat.

The following measures were taken to assess the extent of damage to the 110F and assure its fitness for continued service.

• Hardness readings were taken at those areas which were found to be without insulation. This included the west head, as well as the south west side of the 110F shell. The purpose of this test was to determine whether or not the properties of the material had been altered by exposure to excessive heat. All of the shell plate readings were found to be within acceptable lim- its. All of the head readings were at or slightly below acceptable limits for SA 516-60 material. Ultrasonic thickness measurements taken on this head yielded a minimum thickness of 0.537 in. Using the yield

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Figure 1. Photograph of damaged HOF, 111F, and 112F.

BO OF MPC D- «"-O"*

Figure 2. Photograph of failed RV-105D. Figure 3. Sketch of appropriately supported pressure relief vent stack.

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strength values for SA 516-55, the calculated mini- mum head thickness required for the vessel design conditions are 0.525 in. This leaves a corrosion allowance for the head of 0.012 in.

• All of the externally visible welds were sandblast- ed to facilitate a WFMPT. The purpose of this test is to determine whether or not cracks, which may develop as a result of heat related stresses, exist. No indica- tions were noted.

• All of the internally visible welds were hand cleaned to facilitate a WFMPT. Three small indica- tions were noted at three different level instrumenta- tion nozzles. These were characterized as being origi- nal fabrication defects (porosity and lack of fusion).

All three were excavated until the indications disap- peared via grinding and WFMPT. None were over 1 in. in length. The final depths were measured at 3/32 in, 3/16 in, and 3/8 in. These areas were repaired using an approved welding procedure (SMAW E7018-1 electrodes). The area was preheated to 300°F using electro-resistance heaters. Welding was performed by an individual qualified to that procedure. The area was then stress relieved to satisfy service conditions (ammonia induced stress corrosion cracking) by elec- tro-resistance heaters.

• A visual inspection of the vessel, both internal and external, revealed some minor corrosion under insula- tion (GUI) on the top of the drum. No heat related damage, however, was noted.

• The manway cover bolts and gasket were replaced.

• All of the small bore instrument piping immediate- ly downstream of the vessel nozzle couplings was replaced. This was done in lieu of extensive testing to these systems.

• All of the level, pressure, and temperature instru- mentation were replaced.

• As a final measure, the vessel was successfully hydrostatically tested at 72 PSIG.

• The vessel was recoated and reinsulated with ure- thane foam prior to returning it to service.

111F

The south side of the 11 IF had been exposed to heat resulting from burning synthesis gas emanating from the distorted 105D RV tail pipe. Generally, the foam

glass insulation was intact; however, most of the alu- minum lagging on the south face of the vessel had melted away. All of the instrumentation and associated small bore piping in the immediate area were also found to have been without insulation and exposed to intense heat.

The following measures were taken to assess the extent of damage to the 11 IF and assure its fitness for continue service.

• As there were no exposed sections of shell plate, no hardness measurements were recorded.

• All of the externally visible welds were sandblast- ed to facilitate a WFMPT. No indications were noted.

• All of the internally visible welds were hand cleaned to facilitate a WEMPT. No indications were noted.

• No significant problems were noted following an internal and external visual inspection of the vessel.

No heat related damage was noted.

• The manway gasket was replaced.

• All of the small bore instrument piping immediate- ly downstream of the vessel nozzle couplings were replaced. This was done in lieu of extensive testing to these systems.

• All of the level, pressure, and temperature mstru- mentation was replaced.

• The vessel was re-coated and re-insulated with ure- thane prior to returning it to service.

Piping

All of the lines affected by the fire were visually inspected prior to any cleanup efforts. Any unes miss- ing insulation or with obvious coating damage were slated for additional testing. Material hardness testing was selected as the prime analysis method for evaluat- ing line integrity. Of the lines tested, only a 6 ft sec- tion of a 10 in. ammonia line exceeded its specified hardness level. This section of line was replaced in kind with new material. The two new butt welds were radiographed to verify integrity.

A 2 in. instrument air line, a 3 in. plant air line, a 2 in. 550 psig steam condensate line, and a 3 in. 50 psig steam condensate line were replaced because the lines were badly distorted, apparently from excessive heat.

The piping to RV-105D (Figure 2), its sensing line

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and its tail pipe were all badly damaged. As a result, the RV was relocated and the associated support sys- tem redesigned. These modifications were designed by Piping Analysis Inc.

(Figure 3 shows an appropriately supported pressure relief vent stack.)

All of the remaining lines were either recoated or reinstalled as required.

Structural Steel

A visual inspection of the structural steel was per- formed. Visual distortion was selected as the accep- tance/rejection criteria. The following items were replaced based upon this criteria:

• One 8 in. pipe bent column.

• One 8 in. pipe bent horizontal beam.

• Much of the checker plate at the HOP, 11 IF, and 112F walkway.

• Much of the hand rails and toe rails at the 110F, 11 IF, and 112F walkway.

Relief Valves

The following relief valves were exposed to exces- sive heat and subsequently sent to their respective authorized repair centers for tear down inspections and testing:

• RV-102P;

•RV-110F;

•RV-lllF;and

•RV-112F.

Prior to removal, hardness tests were performed on RV-110F, RV-111F, and RV-112F. All readings were found to be within acceptable limits.

RV-105D was replaced with a new valve from inventory.

Instrumentation

One control valve, three level transmitters, four pressure transmitters and several local pressure and temperature indicators were replaced as a result of obvious or potential fire damage. All of the instrument air tubing, regulators, conduit, and wiring in the fire affected area was replaced.

Electrical

A section of cable tray, along with several runs of conduit, sustained damage as a result of the fire.

These were replaced or repaired as required. Some field splicing was performed. Seven lighting fixtures and their associated conduit and wiring were replaced.

The inspections and repairs described above were completed in roughly 10 days. Mr. Don Hansen of Piping Analysis Inc. was retained to help evaluate the cause of the failure and evaluate other relief valve sys- tems for potential problems.

Recalculation and Redesign of RV Piping and Restraint Structures

This RV piping and its restraining structures were not designed for the actual static + dynamic service.

This is a common failure producing condition in relief valve installations. Proper design must consider all of the important forces that occur, or can occur, from the moment the RV starts to operate to the moment it clos- es and the entire system returns to its beginning state.

This complete cycle must operate within the appro- priate ASME, AISC, or other stress/strain limiting cri- teria without overstress or overstrain.

The best primer on the subject of relief system design is ASME B 31.1, Appendix II:

(1) How to calculate relief valve discharge pipe forces, velocities and pressures.

(2) Differences between open discharge and closed discharge systems, and important considerations for each.

(3) Vent pipe loads.

(4) Dynamic amplification of loads due to support flexibility and earthquake motions.

(5) Stress analysis of the piping and its header con- nections due to pressure + bending loads.

(6) General design considerations such as location, spacing, outlet pipe types, multiple installations, liquid hold-up and water seats, silencers, and supports.

Some important RV forces and restraint features are:

(1) Valve internal mechanical motion, forcing heav- ier air out of the exhaust pipe, initial fluid accelera- tion, fluid steady-state velocity reaction.

(2) Correct time-dependent force vector applied at

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the discharge pipe end cut.

(3) Restraining structural and connecting piping must successfully resist all static + dynamic loadings including dynamic amplifications. RVs must be sup- ported and restrained so that the applied loadings are grounded without causing damage.

• Long discharge pipes require heavy-duty lateral restraints to avoid shaking and columnar failure.

• The RV discharge pipe, very near the valve, is usually properly secured if "anchored," thus providing a good boundary and thrust restraint which does not pass these high loads through the RV or the RV inlet piping.

Piping reactions are usually placed in three main classes:

• Sustained reactions characterized by loadings that persist until collapse, and tend to cause continuous strain, resulting hi irreversible changes and severe damage.

• Displacement reactions include thermal expan- sion/imposed motion, which persist to the extent of displacement. Although limited, detrimental, irre- versible changes/damage can still occur.

• Dynamic loadings which are time dependent caused by forcing functions and can cause irritating noise, severe displacements, and fatigue cracking.

The mechanical connection of the earth, foundation, machinery, and piping defines the scope of structural concerns. The effect of all piping reactions is to cause internal stresses and strains.

Sustained, displacement, and dynamic vectors must be added (superimposed) to produce the total piping reaction at any particular time.

Detecting and Avoiding Excessive Piping Reactions

Successfully detecting and avoiding excessive pip- ing reactions requires two efforts - accurately calculat- ing the actual piping reactions and subsequently designing for an operable system, and proper construc- tion, operation and maintenance.

The requirements for a usable analysis: Accurate calculation - the mathematical model is as close an approximation to reality as the technological method allows - and precision - the number of significant fig-

ures that result from calculation within the scope of the technological method cannot be emphasized enough. It combines competence in mechanical engi- neering, experience with real-world piping problems, and accuracy in both the mathematical model for com- putation and calculation algorithm for computer pro- grams.

Failure-Producing Conditions

As practical as piping computer programs are, they have then- limitations. Without sufficient engineering background, the user does not learn the nature of cata- strophic failures, basic principles of piping behavior, or assumptions made by the computer program. This can be dangerous because it substitutes the program's judgment for that of a person. A few of the failure-pro-

ducing limits from calculations include:

• Hook's Law (stress remains proportional to strain).

Applies to method of elastic analysis with superposi- tion of forces, moments, and stresses used by piping programs; actual stress is not always proportional to strahl in some problems calculated with the stress the- ory of failure used by piping codes.

• Piping systems composed of long and slender members provide the most accurate answers. The use of close-coupled, stiff-to-flexible elements can pro- duce undetected wrong answers.

• Expansion j oints or vessel shells can be improperly described, and explicit or program-assumed stiffness characteristics (strain analysis) can be used without proper stress analysis of these elements.

• Pipe supports such as restraints, hangers, anchors are assumed to work as calculated. The typical and almost universal problem is the failure to include sup- port stiffness and movement hi the piping calculations.

Failure-Producing Conditions:

Construction, Operation, Maintenance

Maintenance must examine the as-built system and compare it to as-designed requirements. A poor under- standing of the nature and significance of system func- tions can lead to failure-producing conditions.

Training programs should include input from engi- neering designer so the total scope is presented to

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those who will build, operate and maintain the real system.

All supporting elements must be checked for evi- dence of intended operation. This includes piping, valves, structures, foundations and anchor bolts.

Substituting different materials of construction with- out engineering investigation is a common error. Start- up, emergency, or normal operating conditions, other than those used for design, are common causes of fail- ure and damage.

Failing to notify a knowledgeable person about a

suspicious condition is another common error.

Vibrations in machinery, equipment, structures and supports can often be traced to relief valve and piping reactions. Sagging beams, broken pipe insulation, sway, noises, and leaks usually mean poor design, construction, operation or maintenance. Investigation of problems is worthwhile.

The incident, response and analysis presented here are based on circumstances that are, at least to some extent, unique. The readers may find the presentation relevant to their own situation, but must realize that each case is different

References

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