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BULLETIN BOARD
QUESTIONS AND ANSWERS
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.
Beta version 0411
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QUESTIONS AND ANSWERS
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.
Beta version 0411
Go to next page