Pump Pressure

[Blobajob88]

Hi,

Could someone please tell me if a pump can exceed it's dead head pressure? i.e. if a pump has a dead head pressure of 4 bar but the pressure coming into the pump is 7 bar, would the pump ramp up the pressure to 11 bar?

 

[hydtools]

Sure. The pump creates a deltaP, inlet to outlet.

Ted

 

[EdStainless]

That is if the case and seals can hold that pressure, I have seen the consequences.
And you can only run a pump at deadhead for a few minuets before you overheat and destroy it.
I used to build multi stage centrifugal pumps, we built pumps that would generate 14,000' of head, but the housings would only contain up to about 10,000' so submergence was required in order to run at full pressure.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Also depends on what type of "pump"

A PD pump will pump until it stalls the motor on closed head or until the pressure relief valve lifts.

As noted above, centrifugal pumps creates a differential pressure / head increase. Also note that if you change the density of the fluid, then the pressure will change but the differential head stays the same. So a pump rated on e.g. gasoline (Sg 0.7) will create a higher differential pressure if you use it for water duty.

So in your case yes, the pump would deliver 11 barg if everything holds together.

Remember - More details = better answers
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[Blobajob88]

Hi,

Thanks for the responses. The pump is a single stage centrifugal pump. The pump is installed in a bypass loop I.e a pipe is branched from the main line, which then goes back into the main line. The pump is installed in the branched line. The water is pumped through the bypass and back round to the main line. The main line has a pressure of 4-5 bar and the pump dead heads at about 5 bar. We’re finding that the pump is starting to leak. What would the pressure going into the pump be?

 

[LittleInch]

The inlet pressure is the pressure in the mainline. Then if your pump discharge valve is closed then the pump pressure on the outlet would be approx. 9 bar. but then you would have no flow so why turn the pump on?

However I don't really understand what is going on here.

If the pressure on the far side of the valve isn't about 9 bar, then the pump will just run off the curve, vibrate and cause all sorts of damage.

If you don't create some sort of back pressure on the pump it won't last very long.

Can you describe a bit more what is going on here? (Diagrams / drawings / pictures always help)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Blobajob88]

Hi,

Thanks for the response. Here is a diagram of the installation. We needed the pump for forcing the water around the bypass loop as we weren't able to install anything in the main line to force it around the bypass loop like an orifice plate or valve to restrict. PS I never designed the system.

Regards

Ben

 

[LittleInch]

Hmmm, I understand the issue, but normally you would use some sort of PD pump because the head losses are so low and all you really need is a certain volume.

Having a pump with a differential head of around 4 bar seems overkill in this set up and my guess is that the pump is operating well on the right hand side or beyond its pump curve so creating all sorts of issues.

The pump seals which are good for 4-5 bar (dead head) should be good enough for 8-9 bar, but it's not a great idea to run it like that for any length of time or number of times. Far better to turn the pump off first.

It might be that whoever designed it didn't understand fluid flow and thought he needed a pump capable of 5 bar to inject flow back into your pipeline (hint: he or she was wrong).

So if you I would measure pressure at the pump inlet and discharge to see what is happening.

If this differential is very low then introduce some sort of flow restriction in this bypass line (control valve, orifice or a section of smaller diameter pipe) in order to make the pump work properly and find out what pressure the seals are capable of withstanding.

And don't turn the pump on when the valve is closed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

LI don't forget that the pump has to overcome the resistance of the reactor

 

[itsmoked]

LI you suggest a restriction, is that in the pump's suction or somewhere in the discharge?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

In this instance you could do either but discharge is much better.

Chicopee, agree but it's difficult to see that being as high as 4 bAr. As usual we have few details to work work.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Blobajob88]

The pump doesn't have a differential head of 4-5 bar, it just dead heads at that pressure. it was sized to pump against a pressure of 2-4 bar (what the line pressure was thought to be). It is effectively a recirc pump, which always tend to be centrifugal. It's never occurred to me to use a PD pump in a scenario like this. Ideally we would have put an orifice or throttle valve in the main line, to force the water around the bypass but that wasn't possible in this situation as it was client's line, therefore the pump was used to force the fluid around the line.

In this situation, I would think that the pump is operating well on the left side of the curve because the pressure is so high i.e. you would think it would be dead heading.

There is effectively no resistance in the reactor. If you were to pump through the reactor into an open tank, you wouldn't see any pressure increase in the pump.

 

[itsmoked]

Blob; I think you have it backwards. The pump is not dead headed. On the contrary, it's essentially running with no head at all. This is because it's trying (futilely) to create a 4bar rise but doesn't probably cause even a 1 bar rise.

I could see it getting pretty hot as it is 'over pumping' mass. If the motor cannot derive cooling from the pumped media then it could be getting hot enough to damage seals.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Blobajob88]

Itsmoked, thanks for the response.

How would you change this set up? We need to do something to create a pressure differential in the bypass line or else no flow would go around the loop? Would you agree? It would just flow through the main line.

What would be the inlet pressure to the pump? If inlet pressure is virtually the same as outlet pressure then is the pump actually working at the right side of the pump as was previously stated by LI? Some one came up with a solution of putting a pressure reducing valve on the suction side of the pump - do you think this would work?

 

[itsmoked]

Is that a three phase motor or single phase? Size?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Blobajob88]

It's a single phase, self priming motor. 1" inlet/outlet. Pumps between 1 to 4 m3/hr

 

[LittleInch]

Ok, now we're getting somewhere.

This is a small dosing pump which, as far as we can tell from here, is running with virtually no back pressure.

If you can advise what the pressure is on the pump suction and discharge we would be in better shape, but lets assume it's less than 1 bar differential. the actual reading on the discharge might be 5 bar, but this is only just above the main pipe pressure.

The pump, although small, is working outside of its normal range and hence the leak ( from where?) might be due to that or just that the inlet isn't supposed to be a 4 bar. Small pump seals are not normally very sophisticated so it could just need tightening or replacing.

Long term you could regulate the inlet pressure down to say 2 bar, but it's probably easier just to insert an orifice plate somewhere between the pump discharge and the entry back into the pipe.

How do you or can you measure flow when it's fully piped up?

Like I said earlier, most of this can be solved next time by using a PD pump of some sort to pump the flow you want without having to worry about pressure. Small dosing pumps are often multi piston or diaphragm pumps.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Blobajob88]

Hi,

Would the inlet pressure to this pump be 5 Bar? and if I was to close the ball valve after the pump (deadhead it) would it ramp it up to 10 bar as the deadhead pressure is 5 bar?

 

[LittleInch]

Yes.
Yes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Blobajob88]

Thanks LittleInch