shut off pressure, minimum flow and related discussion

shut off pressure, minimum flow and related discussion

On 6 January 2012 06:20, farid farid <bachik_syes@yahoo.com> wrote:

i do have 3 questions

1. can i know what is shut off pressure?

from my understanding, shut off pressure is the the higest pressure that can be produced by a pump and this happened at zero flow i.e. control valve closed at discharge line. so when pump running during discharge valve closed, the pressure build up in the pipe line until a maximum pressure and this maximum pressure is shut off pressure. am i right?

2. can pump running when discharge valve closed for a long period of time i.e 1 day? or is this will make the pump damage? from my knowledge, in the pipeline usually they have pressure transmitter which can detect high pressure in pipeline and will trip the pump once the pressure exceed the level. hope can correct me if i am wrong.

3. in order to size a pump, we need to know the total head of the system which is discharge head - suction head. then we get the total head. then how we decide the right pump for that system?

Thank you SLA for these great answers and especially for taking the time. I would however offer the following corrections in red.

Jacques

1. Can i know what is shut off pressure?

• "Shut off pressure" refers to the pressure, when the discharge valve is 'shut off'. Usually this is the maximum pressure the pump will develop. Standards for fire-fighting pump do not desire shut-off pressure (head) to be higher than 1.2 times of rated head. See NFPA 20 for more details

2. can pump running when discharge valve closed for a long period of time i.e 1 day?

• Pump running with discharge valve closed even for 5 minutes will be risky !
• Normally lines of fire-fighting systems are expected to stay fully charged. But 'staying charged' does not mean that the line should be under pressure. It should be full. That is it. In this type of system the line must remain under pressure, because on big systems there are always small leaks, a jockey pump (small pump) is used to keep the system pressurized.
• Usually the non-return valve on the discharge pipe will ensure that water pumped by the pump when it is running will stay in the pipeline even after the pump is shut down. That helps the line to stay charged.

3. how we decide the right pump for that system?

• Two main parameters for selecting a pump are required flow-rate and total head. Both are to be decided by the user according to the system.
• Total head has two parts (1) static head (2) friction head
• If pump has to discharge into a pressurized vessel, e.g. boiler drum, the pressure is to be converted into equivalent static head and to be added to the part (1) static head
• One needs to add velocity head if discharge pipe size is smaller than suction pipe size. But one can add 1 m for that, which is more than enough.
• Static head is usually clear from the plant layout.
• Friction head needs to be calculated from reference charts, tables, etc. For these types of systems, the Hazen-Williams friction formula must be used because the NFPA stipulates that it be used.
• One needs to consider "equivalent lengths" of various fittings and valves

From: S. L. Abhyankar
To: jchaurette@pumpfundamentals.com; jacqueschaurette@videotron.ca
Cc: pumpfundamentals@yahoogroups.com

Dear Mr. Jacques Chaurette,

Both of us have passion to look into problems on pumping, right ? It is that passion, which has motivated you to start this group ! I appreciate that very much.

On another group, there was a post of repetitive failure (breakage !) of the shaft of a jockey pump. I guess, just to maintain the line pressure to take care of leakage, the jockey pump would keep running, even if the flow being pumped is too small, the leakage being small. Jockey pump had a pressure-switch control set at 15.4 bar whereas main pump rating was 12 bar. These pressure-settings seem to be unduly high. Pressure-switch setting of 12.5 bar for the jockey pump should be good enough and at that pressure the jockey pump would be safely away from safe minimum flow. Then shaft-failures would not happen.

Alternatively, instead of pressure setting one could explore using level control, to ensure that the line is full at the topmost point. If the level goes down by 5 m or so, that event should activate the jockey pump. Pump should get cut off when topmost level is reached. That is what I meant by "line should remain charged does not mean it should be under pressure."

Regards,
SLA

Dear SLA, thanks once again for your valuable input. Unfortunately I disagree, the line must remain under pressure to garantee instant coverage of water from the sprinkler(s) head(s) in case of fire. I don`t know why the jockey pumps are breaking, it should be set to start at a given low pressure and stop at a set high pressure with a reasonable dead zone. The pump will probably run for no more than a few minutes and stop on high pressure. If it stops and starts too often there is a serious leakage in the system that must be fixed.

cheers,

Happy new year

Jacques

From: S. L. Abhyankar [mailto:sl.abhyankar@gmail.com]
Sent: Saturday, January 07, 2012 12:38 AM
To: jchaurette@pumpfundamentals.com; jacqueschaurette@videotron.ca
Cc: pumpfundamentals@yahoogroups.com

Dear Mr. Jacques,

As you have observed, the problem seems to be with the pressure setting, how much low pressure and how much high pressure for starting and stopping of jockey pump. And looks like O&M manuals need to clarify what to do when the jockey pumps starts and stops too frequently. As you have observed, there would be heavy leakage in the system, which would cause that.

Also the difference in the low and high pressure settings should be good enough. By that also the frequency of starting and stopping will not be too much.

You have made a good point that sprinklers would need line to be pressurized.

When I made the point of line should be full and not necessarily under pressure, I had in mind the fire-fighting system drowsing fire by water from hoses. Such system may not need line to be under pressure. Pumps would develop needed pressure.

However, sprinklers would certainly need.

So whether line should be under pressure or not, would depend on the type of fire-fighting system. Am I right ?

Regards,
SLA

Indeed you are right.

Jacques