Parallel pumps operation
Parallel pumps operation
(OP)
I've recently integrated a new pump at a boiler feed water plant that is in addition to three existing pumps.
Three old pumps are identical(2 motor driven and one steam turbine driven pumps) with 73m shut off head.
The new pump (Steam turbine driven) is smaller and have 58m shut off head.
When we use 2 steam driven pumps,(new and old) there is no problem, but when the third pump starts it will cause the new pump to work without flow and the pump check valve will fluctuate,and after a while the casing temperature will increase. the cavitation also occurs in the new pump.
I know that we have made a mistake in pump selection, but how can we solve the problem.
How we can change the system curve to use 2 old pump together with new pump?
is that possible to use ARC valve or it is better to use a control valve?
Three old pumps are identical(2 motor driven and one steam turbine driven pumps) with 73m shut off head.
The new pump (Steam turbine driven) is smaller and have 58m shut off head.
When we use 2 steam driven pumps,(new and old) there is no problem, but when the third pump starts it will cause the new pump to work without flow and the pump check valve will fluctuate,and after a while the casing temperature will increase. the cavitation also occurs in the new pump.
I know that we have made a mistake in pump selection, but how can we solve the problem.
How we can change the system curve to use 2 old pump together with new pump?
is that possible to use ARC valve or it is better to use a control valve?





RE: Parallel pumps operation
Presumably the existing pumps were installed with a discharge head that is appropriate for the operation of the boiler. If that true, then you probably need a pump that has a higher discharge head. Perhaps a larger impeller may be installed on the newly installed pump to increase the discharge head.
RE: Parallel pumps operation
As a note of caution, any speed increase on the turbine needs to be carefully analyzed. You need to verify that you will not create a hazard related to the maximum continuous speed of the pump or turbine. You need to verify that you will not create a potential for pressure higher than the downstream system can accommodate. You might also need to adjust the overspeed trip set-point of the turbine. Consult with the turbine manufacturer for all of the possible implications of the speed increase.
Another alternative could be to install an orifice in the discharge lines of any pumps that have a flatter curve to make them behave as if they have a steeper curve. A spreadsheet program could be used to adjust the turbine speed and orifice size to get the best match over the entire operating range that is required.
Johnny Pellin
RE: Parallel pumps operation
Thank you for your early reply,
please find herewith enclosed the pump Curves
RE: Parallel pumps operation
www.pump-zone.com
www.mcnallyinstitue.com
www.pumpsystemsmatters.org
http://www.pumpfundamentals.com/pump_glossary.htm#...
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RE: Parallel pumps operation
I would suggest that the 2 old pumps are running at less than 10.8 m3/hr (each unit) but at a higher head than 54m (probably in excess of 58m) - therefore the new pump is not capable of running against the actual operating head.
You need to establish the flow rate and discharge head on the system with the old pumps operating and then look at the likely changes of flow and head with the 3 pumps in operation.
Bear in mind that putting a 3rd pump into the system ( I guess you are trying to increase the flow rate of the system) you will further increase the discharge head - more friction losses - forcing the old pumps further left on their curve which increases the discharge head.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Parallel pumps operation
If you added an orifice to the new pump, you could probably get a very close match across the entire curve. The orifice plate will make the curve steeper. The speed increase will increase the head at shut-off. In combination, you can achieve the shape you need.
Johnny Pellin
RE: Parallel pumps operation
You have a choice between increasing the speed of the turbine as JJPellin suggested or a larger impeller if that is possible. Either of these proposals will work to increase the pump discharge pressure. The pump supplier should be able to recommend the best alternative.
Neither of these solutions will probably be an ideal solution since the curves do not appear to match. It may not be possible to obtain the maximum flow that you desire which is the reason for the multiple pump operation.
In general, steeper pump curves are recommended for parallel pump operation.
I am not sure that there is much benefit to adding an orifice plate. Adding an orifice will work to shift the pump operating point leftward. Even if you add an orifice plate, the pump discharge pressure still seems to be inadequate to compete with the other pump.
RE: Parallel pumps operation
From your description, the new pump is not able to overcome the head of the old pumps.
I find that it's most helpful to have actual pressure readings of the discharge of each pump, instead of relying on where it was selected to run based on plant design.
RE: Parallel pumps operation
RE: Parallel pumps operation
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Parallel pumps operation
1) You requested a 4th pump with the same design point as the existing three pumps.
a) We all know if you plan to run all 4 at the same time, flow will be reduced
b) This may have made sense at the time if it was to be an "installed spare" to still allow two pump operation if one of the originals is down for maintenance.
2) The selection wasn't reviewed thoroughly for parallel operation
a) Pump vendor did not have enough info, or care enough to review it, or was led to believe that it would actually operate exactly at the "rated point" so deemed the selection acceptable
b) Purchaser did not review thoroughly enough. I suspect it was a case of whipping out the checkbook due to an urgent need for equipment.
c) Both parties could have helped avoid the situation, but keep in mind that all a salesman needs to sell a pump is a design point and a customer's purchase order, the rest is just silly little details.
d) NEVER trust an OEM equipment salesman with an aftermarket/retrofit/ "like for like" replacement job unless you are replacing all of the equipment. If the new stuff has to work with the old stuff, find a dedicated aftermarket sales engineer.
3) The logic for adding another pump may have been flawed, too nebulous to explore without some additional information, but a re-rate at next maintenance opportunity (buy larger impellers and have then on-hand) may have been best.
RE: Parallel pumps operation
I think your point d) is a bit over the top, you are assuming that all OEM salemen don't know what they are doing. I would suggest that there are many OEM "salesmen" who would run rings around some of the so-called "aftermarket sales engineers".
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Parallel pumps operation
Interesting that you comment on 2d but not 2c, if you were to take offense to either, 2c is much more flagrant (and I will admit, maybe a bit over the top.)
RE: Parallel pumps operation
First you have to find out what is the total flow you need for running 3 pumps and recalculate the new total head required. Checked the existing pup curve if it still can meet the new head required.
Specified the new pump flow rate by 1/3 of the total flow and the increased required head with shut off head more less the same as the existing pumps if there is no changes.
There are possible way to make the new pump works with the old pumps in your current situation.
1) Increase the turbine speed or install a bigger diameter impeller to get the same shut of head as the old pumps.
2) Use the affinity law to plot the new curve.
3) Installed an orifice plate at the discharge flange to reduce the discharge pressure /diff.head at rated flow to 54 M
.
RE: Parallel pumps operation
The whole lot is speculation based on not enough information to draw any conclusion.
RE: Parallel pumps operation
You would also know that it is a high pressure boiler because he is pumping DI water.
RE: Parallel pumps operation
RE: Parallel pumps operation
We are still working without a full understanding of the problem and until such times the OP enlightens us with more detail it is still all guess work.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Parallel pumps operation
"If you added an orifice to the new pump, you could probably get a very close match across the entire curve. The orifice plate will make the curve steeper."
I have heard this statement before from someone but I have never understood what is actually means. Add an orifice where...in the discharge? a minimum flow spillback? Can you explain to me what this means?
Thanks.
RE: Parallel pumps operation
Since the pump curve doesn't change, that also means that either a new operating point will be created, one at a lower flowrate then previous, due to the flow restriction of the orifice, or if you can keep the same flowrate, a higher head will now be required to move it. One way to keep the higher flowrate is to speed the pump up, which will also tend to increase discharge head.
Those two combinations, adding an orifice plate plus increasing speed, will tend to deliver higher head at any given flowrate. You will need to size the orifice exactly right, so that those two combinations will not increase the head too much.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Parallel pumps operation
No system curve has been presented in this thread, but it seems that since the pump curve is flat with a lower head then the existing pumps, that there is no way an orifice installation will allow this new pump to compete with the existing pumps. However, if the pump performance curve was steep, an orifice may work because then the curve would move the discharge head above that of the existing pumps.
The addition of an orifice plate is probably more successfully accomplished in a pump system where a single pump is operating. With a parallel pump system, moving the system curve leftward is self defeating because the intent is obtain more capacity.
http://www.engineeringtoolbox.com/pumps-parallel-s...
http://www.engineeringtoolbox.com/pump-system-curv...
RE: Parallel pumps operation
But a similar statement was made saying can we add an orifice? I didn't understand how an orifice would help the situation. The orifice increases the system head pushing the pump further back on its curve and further into the flat portion - where we don't want to be. For the pumps to operate well we need to be further out on the curve - which means system head needs to be reduced.
RE: Parallel pumps operation
Whether it modify the pump curve or the system curve depends on which side of the orifice you are measuring the pressure.
The posting at the below link has a good discussion on this subject and how is it done.
http://www.linkedin.com/groups/SHARING-EXPERIENCES...
I fully agreed with that some of you will argue that it is inefficient way of operating a pump. But then again only if you have better alternative.
RE: Parallel pumps operation
Sorry, but no. An orifice always modifies the system curve. Just like a fixed-position valve. Only a change to the pump, physical change, or speed change, can modify the pump curve.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Parallel pumps operation
This pump manufacturer is correct in the fact, that the addition of the orifice plate (if it is treated as part of the pump) will create a pump curve (pump plus orifice) that is steeper and continuously rising to shut-off. That is the effect I was attempting to duplicate. As many of you already noted, this would also require an impeller diameter increase or speed increase to offset the head lost at rated flow.
Johnny Pellin
RE: Parallel pumps operation
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Parallel pumps operation
Johnny Pellin
RE: Parallel pumps operation
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Parallel pumps operation
Johnny Pellin
RE: Parallel pumps operation
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Parallel pumps operation
If you treat the orifice as the system component with the increased head, some one who is not aware of the history will will end up buying a pump with too high head because the orifice is removed with the pump...and the problem recycle itself.
RE: Parallel pumps operation
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Parallel pumps operation
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)