How to correctly operate positive displacement pumps?

Positive displacement pumps also known as volumetric pumps are very common and they are mainly used for high viscous fluids transfer, like fuel, oil and sludge. Gear pumps, reciprocating pumps, rotary displacement and vane pumps are the most known and common used onboard vessels.

Screw pump type. Source and credit: KRAL GmbH

In these above pumps a special form allows an almost tight engagement of three intermeshing screws 7 and 12 to be achieved. Together with the pump casing that encompasses the screw package , closed volumes are achieved by these means.
The standard direction of rotation of the pump is clockwise seen from the drive (identified by an arrow on the flange cover). When the main screw 7 is driven by a motor, the idle screws 12 follow the main screw due to meshing. The displacement effect of the pump results from the rotation of the screws that leads to continuous filling, axial displacement and discharge of the volumes.

Positive displacement pump working principle. Source and credit: KRAL GmbH

When newly installed or after a major overhaul, before the pump is started, the engineer must make sure that the pump is filled with fluid. This operation is very important because dry running can damage the pump.

Vent plugs on the volumetric pump. Source and credit: KRAL GmbH

Usually, there is a vent hole on the pump casing. If the pumped medium on the suction side is under inlet pressure, open the inlet valve and fill the pump. In the same time you can turn the shaft or the fan impeller of the motor by hand to speed up the filling process. Similar procedure can be applied when the pressure is already available on the pressure side. If there is no pumped medium available at either suction or pressure side, the fill the pump via vent hole. The pump must be filled until the fluid escapes at the vent hole.
After the pump is filled the direction of rotation must be checked. This is usually marked on the pump and as specified before is normally to the right (clockwise) as seen from the motor side.

The pump is sealed at the main shaft outlet by a shaft seal and the most known are the mechanical seals.

Mechanical seal

A mechanical seal contains three sealing points. The stationary part of the seal is fitted to the pump housing with a static seal –this may be sealed with an O-ring or gasket clamped between the stationary part and the pump housing. The rotary portion of the seal is sealed onto the shaft usually with an O-ring. This sealing point can also be regarded as static as this part of the seal rotates with the shaft. The mechanical seal itself is the interface between the static and rotary portions of the seal. One part of the seal, either to static or rotary portion, is always resiliently mounted and spring loaded to accommodate any small shaft deflections or shaft movement.

Mechanical seals require constant lubrication. The inevitable minimal leakage caused by the lubrication of the mechanical seal counts to a few cubic centimeters per hour and is imperative for proper functioning of the seal. The leakage vent holes in the flange cover will allow the drain of this regular leakage.
Dry running must be avoided at all costs , as the seal will overheat and be destroyed in a matter of minutes.

Mechanical seal. Source and credit: KRAL GmbH

In case of mechanical seal failure, the below video is self explanatory with regard to mechanical seal replacement.

Mechanical seal replacement. Source and credit: KRAL GmbH

Excessive pressure can build up in positive displacement pumps if a delivery pipe becomes blocked and the parts under pressure may even break. Therefore the pumps are fitted with an overflow valve (pos. 2) to protect them against this type of problems during operation. Usually the valve is accessible via a plug screw (pos.1) in the end cover and can be adjusted without disassembling the pump.
Normally when leaves the manufacturer’s workshop, the valve opening pressure is set at 110 % of nominal pressure.

Structure of a screw pump. Source and credit: KRAL GmbH

These valves are safety elements and should not be used for pressure control or regulation, like maintaining pressure. If the valve is kept open for too long, under adverse conditions, it will take few minutes for the valve and valve seat to be damaged. As a result, the valve will permanently leak and there will be a reduction in the delivery rate. Similarly, circulation through overflow valve for too long can lead to pump overheating who will reduce the fluid viscosity and can lead to pump failure.

In conclusion, for starting and operating positive displacement pumps the following procedure and precautions can generally be applied:

  • before starting, the engineer must ensure that the pump is filled by opening the vent plug until the fluid escapes at the vent hole as explained above. Never start the pump without priming it first.
  • make sure that suction filter is cleaned.
  • make sure that pressure gauges are functional and calibrated.
  • ensure that, especially on old pumps model, quenching oil level is adequate.
  • engineer must ensure that suction and delivery valves are open. Dry running of the pump can damage its components within minutes. Similarly, running the pump with close delivery valve must be avoided.
  • adequate suction pressure must be always available for proper function of the pump. Proper attention should be paid to vessel’s bunker transfer pump, especially when bunker tanks stripping process takes place, as is highly likely of pump losing its suction and running dry.
  • shortly start the motor to check its rotation, especially for newly installed pump or after a major overhaul. If the direction is wrong, change the motor phase connections and try it again.
  • the pump should be operated with at least 1.5 bar differential pressure to ensure that internal components are properly lubricated by the pumped fluid.
  • systems where positive displacement pumps are used must be equipped with a safety device like pressure regulating valve and engineer must ensure that the valve is properly working and adjusted. The pump and system should not be used without or with defective pressure regulating valve.
  • the pump should not be operated outside its designed specifications.

Engineer’s proper care and regular service maintenance as per manufacturer’s instruction manual will ensure a long service life of the pump without being necessary to intervene for repair and troubleshooting. Engineers must be familiar with pump’s operation and monitoring and during their regular engine room rounds must ensure that:

  • pressure gauges are functional.
  • pumps are running within designed parameters.
  • quenching oil is at the correct level.
  • there are not abnormal sounds or vibrations during operation.
  • the pump is not overheated.
  • there are no abnormal leaks present.

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