With reference to fire or explosion explain the significance

of the following properties of a flammable gas; (a) combustion pressure,

(b) explosive limits, (c) flashpoint,

(d) density.

5. Sketch the construction and describe the operation of the following types of fire detector;

(a) vapour products (ionisation), (b) flame sensor (infra-red), (c) heat sensor (rate of rise).

Explain why use of all three types together is to be preferred to the use of one of these types alone.

PUMPS AND PUMPING SYSTEMS 35 1

develop-especially in the case of the reciprocating pump-high pressure differentials if required.

2. Dynamic pressure pumps (or roto-dynamic pumps), in which a tangential acceleration is imparted to the fluid. These

CHAPTER 9 include centrifugal, axial and mixed flow types (the latter is part

axial, part centrifugal). Depending upon supply head they may require a positive displacement pump as a priming device.

In general they would be used for medium to high discharge

PUMPS AND PUMPING SYSTEMS

rates, they usually are confined to low viscosity fluids and

generate only low to moderate pressure differentials. A ship's engine and pump room obviously contains a number

of complex pipe arrangements. Bilge, ballast, oil fuel, sanitary Reciprocating Pumps

water, etc., involving numerous pipe leads, cross connections, There are numerous forms of such pumps, both horizontal valves and so on. The pumps and equipment provided on a and vertical, used for all duties on ships. The reciprocating modern ship for these various duties are of all types and sizes. motion can be through connecting rod from electric motor Obviously then a description under the above heading must be drives or other forms, but still one of the most common forms is

drastically cut down and simplified. This chapter then attempts the direct steam drive.

to simply pick out what are regarded as essential units or pipe

groups and present them in a form suitable for examination The Weir Steam Pump

needs. The work may seem somewhat disjointed but it is The steam end consists of a forged steel piston with either assumed that the student can utilise his background practical Ramsbottom type plain spring rings or type spring experience to join the various sections together towards a restriction rings, rings of cast iron with a vertical lateral

comprehensive picture. clearance for the plain rings of about 70 pm and a butt gap

Firstly the main types of pump in regular use are considered clearance of about 140 pm when fitted. The cast iron steam together with any relevant points. Associated equipment is then cylinder has steam top and bottom from a bolted on valve chest considered, oily-water separators, feed water injectors, etc. which is provided with the required steam and exhaust valves Lastly some pipe arrangements and fittings are presented, and drain cocks. A lubricator for hand use is usually provided, concluding with an attempted concise grouping of the rather filled with cylinder oil and graphite, but for superheated steam lengthy rules and regulations appertaining to pumping systems. mechanical lubricators are usually used. A mechanical stroke It should be stressed that these rules do not have to be counter is also sometimes fitted. The steam piston is bigger in memorised and are given only to allow the student to have some diameter than the bucket to allow the pump to work at lower

of the minimum requirements. steam pressures than the discharge feed pressure (principle of

differential areas) and as numerous sizes of pump are used the

TYPES OF PUMPS given sizes and clearances quoted here are based on a 150 mm

bore pump, bigger pumps having proportionately larger

Classification clearances. A nickel steel piston rod and a brass or bronze

1. Positive displacement pumps, in which one or more bucket rod connect by screwing into a main steel crosshead and chambers are alternately filled then emptied. These include are locked by a steel taper pin. The steel valve gear levers have reciprocating, screw, gear and water-ring types etc. They do not the fulcrum on a front stay and slide through a crosshead pin require a priming device, in fact they may be used as priming during vertical movement of the crosshead. The steel valve gear rod works off a ball crosshead between fulcrum and main In general they would be used for small to medium discharge crosshead, the lever movement butting against top and bottom rates, they can pump fluids of a wide range of viscosity and can adjustable nuts to transmit motion to the valve rod.

352 REED'S GENERAL ENGINEERING KNOWLEDGE

_I

PUMPS AND PUMPING SYSTEMS

produces ridges in the working bores. To adjust the strokes the valve spindle is screwed up until the piston is striking the top cover and then screwed down and locked to allow the piston to approach to within say 12 mm of the end cover. The process is repeated using the bottom nut and lock nut on the threaded spindle for the cylinder bottom.

The water end is of cast iron with a bucket working

in a brass or brone liner (cast iron throughout is used for oil pumps). The bucket usually has two grooves into which are fitted special ebonite (or tufnol) rings, the lateral clearance being

about and the butt gap clearance about The rings

are cut and then heated in boiling water to make them flexible, the butt gap being adjusted by trying in the liner bore. When the correct butt gap is made the rings are cooled whilst sprung open to 9 mm butt gap so that the ring when fitted has the necessary

compression. The double acting chamber has a twin unit valve chest at the front, each unit, one top and one bottom, having a suction and discharge valve set. The valves are spring loaded from valve guard plates, smaller old pumps usually employ flat

brass plate type valves but modern larger pumps

almost always employ group valves. Such valves are small circular valves, about five or seven in number, in a circular pattern, the valves being spring loaded from the guard plate. For heavy duty, say for example hot feed water, etc., the valves and seats are of stainless steel and are of the flat faced type. Each valve chest is usually provided with a small sentinel type relief valve on the top covers.

It is of course a requirement that a relief valve is fitted in the discharge pipe irrespective of the cover valves. The pump is also fitted with air pet cocks, drain plugs, air vessels, float control devices, suction and discharge valve chests, etc., as may be required for its duty, horizontal and vertical types are available for feed, oil fuel, ballast, bilge and service duties.

The Weir Type Valve Gear

The valve spindle driving rod is connected to a flat plain outside steam slide valve which works on, and is carefully bedded to, the flat back face of a round shuttle valve which distributes cylinder steam and exhaust. The slide valve, or auxiliary valve, has a vertical motion and the shuttle valve has an axial motion. The auxiliary valve is adjusted by liners so that with the auxiliary valve bolted up in place the shuttle valve can be lightly tapped across by hand. The shuttle valve works at its ends inside hollow bells, the bells being a smooth sliding fit over

the shuttle. The bells have a slot across their back face into which slots a tongue piece from the end cover, the tongue piece

I can be rotated by a nut in the centre of the end cover (externally)

which serves to turn the bells. The tongue piece spindle has a pointer outside the end cover, the pointer can be moved between two stops by moving the adjuster nut. Rotation of the bell serves to vary the opening to the bypass port so altering the quantity of extra steam supply. The inner face of the bell is bedded to an inner circular web, provided with slot or slots cut in to allow

I steam t o pass into the cylinder formed between shuttle valve and

I bell at the end. With the end covers bolted up tight the bells

I

should have an axial clearance of about of the bell by the adjuster nut when the pump is working. to allow rotation The left hand pointer points up usually as it controls the pump upstroke while the right hand end indicator will point down.

The bells should only be used for starting the pump. When they are turned t o the open position (indicator to letter cast on the end cover) steam can be admitted into the cylinder at any point in the stroke. When the pump is started the bells may be closed by turning them t o the shut position (indicator t o letter 'S', cast on the end cover). In this way the expansive property of the steam will then be utilised as cut off will take place at approximately stroke, this provides an economy. The bells must not be left open during normal running.

It is essential for all reciprocating pumps to maintain valves and rings, etc., in good order, this is applicable particularly to the shuttle valve gear. Regular skilled examination and attention can give efficient and reliable pumps.

Centrifugal Pumps

Impeller and Volute Casing

In single stage pumps a single impeller rotates in a casing of spiral or volute form and in multistage pumps two or more impellers are fitted on the same shaft. Fluid enters the impeller axially through the eye then by centrifugal action continues radially and discharges around the entire circumference. The fluid in passing through the impeller receives energy from the vanes giving an increase of pressure and velocity. The kinetic (velocity) energy of the discharging fluid is partly converted t o pressure energy by suitable design of impeller vanes and casing. In some pumps turbine driven boiler feed pumps, diffusers are used. These consist of a ring of stationary guide vanes surrounding the impeller, the passage through the diffuser vanes is designed to change some of the velocity energy in the fluid to

REED'S GENERAL ENGINEERING KNOWLEDGE PUMPS AND PUMPING SYSTEMS

pressure energy. In double inlet pumps fluid enters from two required duty, head to lift, head to discharge (pressure),

sides the impeller eye as if there were two quantity, etc. A typical centrifugal bilge pump would give an

back giving twice the discharge at a given head. output of about 3 0 kg of water in one second, 12 power,

the fluid from one impeller is discharged discharge up to 5 bar running at about 17 The casing

passages the eye of the next impeller so that usually has the suction and discharge branches arranged at the

(or discharge pressure) is the of back so impeller and spindle can be removed from the front

the number of stages, such a pump is without breaking pipe joints. The discharge branch is usually on

discharge at moderate speed turbo-feed). the pump centre line so that the pump is not 'handed'. The

i _{number of impeller vanes is not 'fixed but usually there are six to}

ten. The volute casing is like a divergent nozzle which is wrapped around the impeller and serves two main functions it enables velocity energy to be converted into pressure energy, the degree of conversion is governed mainly by the degree of diverence (2) it accommodates the gradual increase in quantity of fluid that builds at discharge from the circumference of the impeller. For the velocity to be constant the volute is made so that cross

I sectional pipe area increases uniformly from cut water to throat.

With an impeller having six vanes then the cross sectional area of volute at No. 1 vane will be of throat area as one vane is pumping of the water quantity, similarly at No. 2 and so on, taking vanes in turn from cut water to throat. If the discharge were choked or blocked then the pump would merely churn water so that the fitting of a relief valve is not essential. A common fault for repair with these pumps is the increase of clearance due to wear at the bearing rim (or sealing ring) faces. This allows connection between suction and discharge so drastically reducing efficiency. On the larger pumps these faces are often brass strips on liners secured by countersunk screws, clearance adjustment is effected by adding further liners. On the smaller pumps the faces are made by sealing rings which are renewable. After any overhaul or clearance adjustments, care must be taken on re-assembly to make sure motor or impeller are not pulling on each other at the junction coupling.

In the smaller designs the shaft gland seal is by an ordinary stuffing box, water cooled, usually employing lead foil type packing. Great care must be taken on these packings as they are very prone to nip and score the shaft severely if not properly

double

Fig. 9.1

impeller and single and rings are free to slide along the shaft under the action of

together with a volute casing axial springs from the clamp ring. The cup ring presses on to a

REED'S GENERAL ENGINEERING KNOWLEDGE PUMPS AND PUMPING SYSTEMS

plate which bolts to the pump casting. Grease lubrication is this would affect the choice of material (although for salt and provided to the face between ring cup and fixed ball ring, fresh water the difference is often just the casing).

worked by spring or water pressure. Materials for salt water could be, casing-gunmetal (cast iron

Fig. 9.2 shows diagrammatically a vertically arranged single for fresh water), impeller-aluminium bronze, shaft-stainless inlet centrifugal pump. This is arranged with the casing split steel, casing bearing ring seals-leaded bronze. Space points: vertically one half has suction and discharge branches so that the with vertically arranged pumps less floor space is required, this impeller and shaft can be removed without breaking pipe joints. usually means that no hydraulic balance is necessary, impeller The impeller has a single eye (inlet), upward facing so that air access is simple and no pipe joints have to be broken.

locking is eliminated under operating conditions. Pressure in the A typical engine room pump could be a vertical, in-line,

space under the impeller ensures hydraulic balance. overhung suction and discharge pipes are in a straight line

and the impeller is supported, or hung, from above), either base or frame mounted. From which the impeller can be removed without splitting the casing, breaking pipe joints or removing the electric motor.

Power supplied to the pump must take into account the various losses, these are made up to:

1. Friction loss in bearings and glands, surfaces of impeller and casing. Some impellers are highly polished to minimise friction loss.

VERTICAL, SINGLE INLET, CENTRIFUGAL PUMP (DRYSDALE)

Fig. 9.2

This depends mainly upon duty and space available. Duty points: (1) Flow and total head requirements. This will govern the speed of rotation, impeller dimensions, number of impellers and type single or, double inlet (2) Range of temperature of fluid to be pumped. If suction capability is insufficient to accommodate supply conditions due for example to high inlet

temperature cavitation can occur (3) Viscosity of the medium to _{CHARACTERISTIC CURVES}

REED'S GENERAL ENGINEERING KNOWLEDGE PUMPS AND PUMPING SYSTEMS

2. Head loss in pump due to shock at entry and exit to _{Fig. 9.4 shows such a pump. Its casing would be cast iron or}

impeller vanes and eddies formed by vane edges. _{gun metal. Impeller, aluminium bronze. Guide vanes, gunmetal,}

3. Leakage loss in thrust balance devices, gland sealing, _{these guide water without turbulence to the discharge. Pump} clearances between cut water and casing and bearing seals. _{shaft, stainless steel with solid and flexible couplings driven, if}

A characteristic curve for a centrifugal pump is obtained by _{low head, by a relatively small prime mover at higher speeds} operating the pump at rated speed with the suction open and the _{than a comparable centrifugal pump. A water cooled thrust of} discharge valve shut. The discharge valve is then opened in _{the tilting pad type is required because of the considerable thrust} stages to obtain different discharge rates and total heads (can be _{generated (consider a propulsion system).}

measured by discharge pressure gauge, suction head constant) _{The mechanical seal is water cooled as is the composition bush} corresponding to them. A typical curve is shown in Fig. 9.3. for the shaft. The latter is via the multi-leaf filter, in the case of reciprocating and axial pump curves given for comparison. _{condenser circulating, because of the possible ingress of sand.}

Failure to deliver caused by loss of suction may be due to, insufficient supply head, air leakage suction pipe valve

open on empty bilge, of priming facility or leaking not b e operated

shaft gland. Capacity reduction could be the result of a damaged sealing ring, leaking gland, obstruction (valve partly closed),

incorrect rotational vibration may be caused by

either (1) loose coupling (2) loose impeller (3) bearing damaged (4) impeller imbalance.

Axial Flow Pump

When large capacity, wide variation of low lift head at constant speed, conditions have to be met the horizontal or vertically arranged axial pump is the most suitable.

The pump is efficient, simple in design and is available in a wide range of capacities. It can if required, be reversible in

operation between motor and pump would be

_-

absorbed k

required) and is ideally suited to scoop intake for condensers as it offers very little resistance when idling.

gland and thrust water

Fig. 9.5

Characteristic curves for an axial flow pump are given in Fig. 9.5. Careful study of these curves and comparison with those for a similar speed centrifugal pump given in Fig. 9.6 will greatly assist the reader to answer some of the questions asked, throttling the discharge valve, its effect on pressure, efficiency

duty with cryogenic carriers is shown in Fig. 9.7.

AXIAL FLOW PUMP It is fitted, in this case, with a scroll (or screw) type of inducer

In document Reeds-8 General Engineering Knowledge for Marine Engineers. (Page 180-186)