Nagoor meeran.H: CONTROLLING PD PUMPS -part 2

CONTROLLING PD PUMPS

MACHINE PROTECTION. The greatest danger to positive displacement pumps is overpressure. The rigid, unyielding nature of the pump characteristic means that overpressure is certain if the discharge is blocked. Many smaller (non API) pumps^{1, 2, 3}, such as the gear pumps used to supply lube oil for larger equipment, have integral relief valves to release pressure from the discharge back to the suction. In the majority of cases, an external relief valve must be supplied by the user. It must be connected as closely to the pump discharge as possible and must not have any means of blocking either its inlet or its outlet. It should discharge back to the pump supply. If, for any reason, the discharge is blocked and the relief valve is not capable of relieving, the pressure will rise very rapidly until something busts. It may be connecting rods, the check valves or even the cylinder head. Don't count on the motor stalling because events unfold very rapidly and the inertia of the system is sufficient to cause major damage. The most likely point of failure is the bolting on the discharge flanges.

Direct acting pumps, such as those driven by compressed air, may not need a discharge relief if it can be shown the maximum pressure of the driving fluid is incapable of causing excess pressure.

It is often advisable to install a high discharge pressure shutdown switch or transmitter in addition to the relief valve.

Good engineering practice dictates that operating controls be provided to avoid shutdowns or relief valve operation for normal operating situations. If it is possible for the pump discharge to be blocked under normal operating conditions, a pressure control loop must be provided on the discharge. This consists of a pressure transmitter, a controller and a recycle valve. If there is already a flow control loop on the discharge, a pressure override controller must be added. A common arrangement is shown in Figure 2-8. A deviation alarm on the pressure controller provides the pre-alarm for the high pressure shutdown. Whenever the pressure is above the setpoint of the controller, the alarm is on. This has the advantage of having only one setpoint for the two functions. Since the valve is fail open and the lower of the two signals drives the valve to the safe state, a low selector is chosen to pass the correct signal to the valve. Once again it must be stressed that overpressure conditions can arise extremely quickly. All components of the system must be selected with speed in mind. DCS controls with a scan rate slower than ½ second may be too slow. In any case, the valve may be too slow. Despite your best efforts it may be impossible to limit the pressure rise. In such cases it may be necessary to eliminate the high-pressure shutdown and to accept occasional relief valve action.

The suction side of the pump may also require protection. A relief valve is required unless all suction piping is rated for the full discharge pressure. Liquids, especially water, are quite incompressible. Even the smallest reverse leakage through a check valve can raise the pressure of a blocked suction sufficiently to rupture the line. This can happen even after the pump has been shut down! The discharge dampener will contain liquid at full pressure unless it has been relieved. The line rupture may occur minutes or even days after the pump has been shut down and isolated, depending on the relative sizes of the discharge and suction dampeners and the leakage rate. (Been there, seen it.)

A low-pressure shutdown switch or transmitter is required on the suction side of larger pumps. The NPSHR of reciprocating pumps is further complicated by what is termed the "acceleration head". (See the previous article in this series, Controlling Centrifugal Pumps¹, page 7, for a more detailed discussion of NPSHR and NPSHA. Note that there is one difference between NPSH for centrifugal and PD pumps: For a PD pump NPSH is specified in pressure units instead of elevation. This is because the operation of PD pump is not dependent on liquid density. ) When the piston of a simplex pump begins its intake stroke, the liquid in the suction line is essentially stationary. The entire line contents must be accelerated rapidly to its maximum velocity, approximately three times the average velocity. There are two reasons for this three to one ratio: Firstly, the liquid isn't moving at all for half the cycle. Secondly, even when it is moving the velocity starts at zero and builds up to a maximum at mid stroke before reducing to zero again at the end of the stroke. The "suction" required to draw the liquid into the cylinder reduces the pressure sufficiently that air or vapour bubbles may develop. When these collapse during the discharge stroke, if not sooner, cavitation occurs. If the bubbles do not collapse, as in the case of air dissolved in water, serious hammering can occur in the cylinder. The air may accumulate to the point that the pump becomes vapour locked. Remember that air can compress into the internal clearances of the cylinder and then expand again on the intake stroke without ever being forced out of the discharge check valve. The low suction pressure shutdown device should be accompanied by some sort of pre-alarm. Acceleration head problems are greatly reduced for multi-cylinder pumps. Suction dampeners also contribute to making the flow rate more even.

Minor mechanical failure in PD pumps can cause significant vibration and subsequent serious damage to the entire machine. For this reason it is the rule to include a vibration switch on larger equipment. This switch need not be the extremely sensitive, multichannel system used on high-speed machinery. We are not monitoring the gradual deterioration of delicate bearings. What we are looking for is an abrupt event of considerable magnitude. Even the simplest switch will suffice. The usual type of switch is termed a "seismic" switch. It works by having a small weight held in place by a magnet against the force of a spring. A "bump" dislodges the weight from the magnet and allows it to open the shutdown contact. The usual means of "calibration" is a light whack with a hammer. A pre-alarm is not possible.

Larger PD pumps may have special lubricating requirements for the cylinders. The oil is supplied by small reciprocating injectors (miniature PD pumps) drawing from a small reservoir. The reservoir needs a low-level alarm which should also inhibit startup. A shutdown may not be necessary since damage from low oil level is not immediate. The reservoir is supplied from a larger lube oil tank through an integral float valve. The tank requires a low and a high level alarm. These can be provided by a single transmitter.

Variable speed pumps, especially those driven by engines, may require an overspeed trip. This should come from a separate sensor from the governor since it may be a governor failure that has caused the overspeed. A simple method is a small bolt mounted in a hole in the rim of the flywheel and held in place by a spring. Centrifugal force causes the bolt to project from the rim and trip a limit switch mounted on the frame.

SAFETY. There are no inherent dangers associated with PD pumps other than extremely high pressure or leakage of toxic or hazardous materials. Actually diaphragm pumps are especially suitable for toxic service since they have no rotating or sliding seals. The possibility of leakage or even rupture and a subsequent fire must be considered whenever flammable materials are being handled.

It is possible that a diaphragm may rupture during service. If the liquid is particularly hazardous, a double diaphragm may be used. In that case a tap will be provided by the manufacturer to install a pressure sensor for alarm or shutdown.

A fire safe block valve is needed on the suction whenever flammable liquids are being drawn from a reservoir with significant capacity. Its interlocking must be handled slightly differently from that associated with a centrifugal pump. It is not advisable to slam shut the suction valve even if the pump is stopped simultaneously. Full vacuum may be induced during the rundown. If this causes air to be drawn into the piping an extremely hazardous situation is created. It is best to use a time delay circuit so that the suction valve is not closed until several seconds after the pump has been tripped.

It may also be desirable to have a fire safe block valve on the discharge. Since most PD pumps are in high-pressure service, there may be the potential of pressurized fluid forcing its way backward past the discharge check valve into a fire. Automatic closure should also be interlocked to occur at least several seconds after the pump has been turned off.

ACCESSORY INSTRUMENTS. Any instrument used to control the process or to provide some safety or machine protection function should, if possible, have a simple local device to verify its operation. In the case of PD pumps that means pressure gauges at both the suction and the discharge. Pressurized pulsation dampeners require pressure gauges to ensure that they are properly charged. Large reciprocating pumps have oil filled crankcases. A gauge glass (by vendor) and a thermometer should be provided.

The cylinder lube reservoir requires a sight glass. This is supplied by the vendor on API pumps^{1, 2, 3} . The tank needs a level gauge glass whose span is broad enough to cover both alarm settings.

If the machine is equipped with cooling water jackets, there should be a thermometer on the outlet of every jacket. A single thermometer on the supply is a good idea. High outlet temperatures may not mean the pump is overheating!

The variety of PD pumps implies a variety of special requirements. Be sure to discuss these with the pump vendor to make certain that nothing "obvious" has been overlooked.

PARALLEL PUMP INSTALLATIONS. PD pumps are quite suitable for parallel operation. Since the discharge pressure of each pump rises as necessary, all pumps will discharge into the common header. A common recycle valve is sufficient for flow or pressure control.

Starting up a pump that is discharging into a header that is already pressurized by other pumps may overload its driver. To prevent this it is necessary to have an individual recycle valve on each pump. This may be a slow acting ball valve. Starting the pump then becomes a simple timed sequence in which the valve is first opened, then the pump is started, and finally the valve is closed again. The pump should also be shut down in the same sequence. Remember that the ball valve will be opening against the full discharge head and may need a large actuator. In water service it is extremely important that the appropriate water resistant grease is used.

If variable speed pumps are used, the majority should be placed on fixed speed. One pump is then selected for process control to take the swings in demand.

SERIES PUMP INSTALLATIONS. PD pumps are not generally installed in series. Since series pumps must both discharge an identical flow and both are discharging a "constant" flow, it is extremely unlikely that the two can be matched without complex controls. It is common, however, to have one or more parallel centrifugal pumps servings as boosters to one or more parallel PD pumps. The centrifugal pumps serve to provide the NPSH that the PD pumps require. The PD pumps in turn can provide a very high discharge pressure.

The centrifugal boosters should have sufficient flow capacity to supply the pulsating requirements to the PD pumps. This means the full peak flow, not the average. If they need controls they should be on pressure control by way of a recycle valve since there should be no interference in the suction to the PD pumps.

A warning: It may happen that the PD pump has a very low discharge pressure for some reason -- perhaps the piping has been removed for maintenance. It is then possible for the booster pump to push liquid through the various check valves and out the discharge without the PD pump being turned on at all. In fact, the flow may be even greater than if the PD pump were running!

SUMMARY. Figure 2-9 shows a typical arrangement for a positive displacement pump application. The following features are illustrated: