This section contains postings of short questions and answers on a sundry of topics that do not require full-length article write-ups due to the limited information provided by the questionnaires. Readers are encouraged to comment on the Q&A. If enough additional information is received on a specific Q&A, the topic will be rewritten as a separate article that will be listed in our sitemap.
PD pump is series with centrifugal pumps
Q - We are working on a planned pipeline consisting of several pump stations hundreds of miles apart. Due to the low initial temperature and high viscous product conditions at the start of the pipeline we plan to use positive displacement (PD) pumps for the first station, and centrifugal pumps for all other stations. To cut down on cost, the PD pumps will feed directly into the suction nozzles of the centrifugal pumps – there will be no separate containment tanks. Should there be any concerns about operating PD pumps in series with centrifugal pumps?
A - The proposed arrangement is acceptable provided (1) that pressure relief valves/ automatic shut-off valves are provided to protect the pipeline from being over-pressurized during upset conditions, and (2) that pressure pulsation dampeners, or pressure surge chambers, are provided between the PD and centrifugal pumps. Without dampeners the pressure pulsations from the PD pumps will be transmitted, and will likely cause vibration and thrust problems, to the centrifugal pumps.
NPSHR curve and minimum flow
Q - In a typical pump performance curve, is there a correlation between the start of the NPSHR curve and its minimum stable flow? In other words, can we presume that it is safe to operate a pump within the capacity range covered by the NPSHR curve?
A - It is risky to make that presumption. In many specifications, such as API 610, the vibration acceptance criteria is so stringent that they can only be met when the pump operates close to its BEP. Many types of liquid, such as those with dissolved or entrained gas, require that the pumps operate close to their BEP, or the pumps will likely develop high vibration levels. In most cases, the range of an NPSHR curve has no correlation to a pump’s allowable operating flow range.
Direction of axial thrust balancing
Q - When balancing the hydraulic axial thrust of a pump, should it be balanced such that the direction of the residual, or net, axial thrust acts towards, or away from, the thrust bearing?
A - It should be balanced such that the hydraulic axial thrust puts the shaft in tension to minimize the shaft deflection. Subjecting the shaft to compressive stress has a bending effect that tends to increase its deflection. This is especially true with horizontal pumps whereby the shaft deflects more due to the combined effect of static and hydraulic radial loads. In vertical pumps the shaft deflection is less because its static load puts the shaft in tension. Vertical pumps should be balanced with a net thrust in downward direction, or downthrust.
Impeller balance holes
Q - I noticed that single suction impellers, used in single stage pumps, have holes drilled in the shroud located between adjacent vanes near their inlet. What are these holes for? I do not see similar holes drilled in single suction impellers used in multistage pumps, or on double suction impellers.
A - Those are balances to reduce the pressure on the back side of the impeller. The balance holes are drilled to balance the pump hydraulically in the axial direction. Single suction impellers used in multistage pumps are typically balanced by changing the balance diameter of their wear rings, and not by the balance holes because the leakage flow through the balance holes affects the pump performance to a certain extent.
Double suction impellers have no balance holes - the impellers are presumed to be hydraulically balanced because one side of an impeller is a mirror image of the other side. (Caution: this presumption is proven wrong by numerous axial thrust tests that consistently showed that double suction impellers are, in fact, not hydraulically balanced.)
Minimum pump speed
Q - Why would anyone be concerned about the minimum allowable speed of a pump? I understand why one should be aware of its maximum allowable speed, and this has to do with its maximum safe working pressure, allowable shaft stress, NPSHR, critical speed, etc. But what is the deal with minimum speed?
A - At low speed a pump may not develop enough differential head to provide lubrication to its wear parts, the shaft peripheral speed may not provide sufficient lubrication to its journal bearings, and the internal seal flush may not provide ample vapor suppression pressure to its mechanical seal (if any of these items are provided in the pump.) The pump speed encroaching into its critical speed could also be a problem but very unlikely. For these reasons, identifying a pump’s allowable minimum speed is as important as its maximum speed.