Pump selection 1

[Mechh007]

Hi friends,
I have to choose a right pump for my project. I will explain about the task. we need to provide emergency wash shower and eye wash station on the monkey board in a jack up drilling rig. At present we have a pump which delivers 23GPM @ 3.6bar. The pump outlet has been connected to pressure set inlet. The pressure set capacity is 100liters(26.42 GPM), Maximum operating pressure is 145psi, pre-charged pressure is 29psi(2bar).The pressure set outlet is connected eye wash/shower. My question is what will be the pressure at pressure set outlet. My understanding is Pressure at pressure set outlet = (Precharged pressure + pump discharge pressure into pressure set)=(29+52.2) =81.2 psi(5.6 bar). am i right? Pressure required at eye wash station is 14.5 psi(1 bar). The pressure loss between pressure set outlet and eye wash station is 70.61 psi(4.87 bar). which means,
Pressure available at eye wash station = Pressure delivered from pressure set - pressure lose upto eye wash
= 81.2 - 70.61
Pressure available at eye wash station = 10.59 (0.73 bar)
Pressure available is less than the required. so can we enhance the pump capacity? PLease advice.

Regards,
BN

 

[chicopee]

You should show us an elevation sketch because I am somewhat confused by your terms; for example "... The pump outlet has been connected to pressure set inlet.The pressure set capacity is 100liters(26.42 GPM), Maximum operating pressure is 145psi.." the pressure set inlet to what? the pressure set capacity of what? Maximum operating of what?

 

[Mechh007]

Chicopee,
I have attached a hand sketch with pressure set data sheet. Please have a look

 

[Mechh007]

sketch

 

[TD2K]

Is this a centrifugal or PD style pump? If it's a centrifugal style pump, is the 3.6 bar the differential pressure it's designed to produce? What is the suction pressure to the pump?

What is the water flow requirement for your safety shower?

PD style pumps will be specified to produce so much flow at a discharge pressure but in reality, they will produce water pressure the system requires subject to motor limits, design limits, discharge PSV setpoint or internal PSV setpoint. Centrifugal pumps are designed to produce a given differential head/pressure at the design flow so the discharge pressure is this differential pressure plus suction pressure. As the actual flow rate increases/decreases relative to design, the discharge pressure typically decreases/increases respectively (some centrifugal pumps can have very flat curves over a wide range of flow rates).

The 45m elevation change equals to about 4.3 bar if I've done the conversion correctly so your pump has to be capable of putting out at least 5.3 bar pressure to delivery water to the safety shower at 1 barg. On top of that, you need to include the line hydraulic losses, those shouldn't be too much for 20+ gpm in a 2" line.

The surge tank doesn't increase the pump's discharge pressure, it only helps to even out pressure swings on the system.

 

[Mechh007]

TD2K,
yes it is centrifugal pump, Discharge pressure is 23gpm @3.6 bar, Suction pressure is 2.06 bar(30psi). i have attached hand sketch. pls refer

 

[TD2K]

The sketch didn't make it but from your dP and suction pressure, the discharge pressure from your pump will be 5.6 to 5.7 barg. That's slightly above the value I estimated above as being the minimum required. Have you checked the discharge pressure in the field?

Have you checked line losses in your system?

What is the water flow the safety shower is designed for?

I think your surge tank isn't doing you any good. These are usually designed with a pressure switch to start/stop the supply pump. In your case, you need your pump to start at a minimum pressure of 5.3 barg so with your elevation, the pressure at the inlet to the safety shower. You'll have to see what the maximum pressure from the pump is and the stop setting for the pump has to be less than that (or the pump will never stop running).

Is it possible to install a larger impeller in the pump? From the information you've posted, the pump is marginally sized for your system because of the significant elevation change.

 

[Mechh007]

Yes. i checked the losses from pressure set outlet to shower ..the loss is 4.8bar. The required pressure at the eye wash/shower is 1bar. As per the mentioned pump discharge we can have only 0.8bar in the eye wash. Am i right? (5.6 - 4.8 =0.8). So surge tank will not help to increase the pressure..am i right? if we want to have 1 bar pressure at eye wash we need to enhance our pump capacity as 5.8 bar. am i right? Please advice

 

[Mechh007]

If we increase our suction pressure from 30 psi to 32 psi, we can have 5.8 bar at discharge know..

 

[TD2K]

Raising the suction pressure is always an option. How dependable is the suction pressure? Safety showers are one of those things that I prefer not to design on the edge though it's true that if the pressure is somewhat less than 1 barg, you'll still get water to it (even at 0.5 barg, you'll get about 70% of the water flow at 1 barg).

 

[Mechh007]

Thanks TD2K.so we can go with same pump by increasing the suction pressure .am I right?

 

[Latexman]

I did a safety shower (SS) rejuvenation project a few years ago on a unit built in 1957. About 50 SSs total. A lot of the older pipes had internal build-up, and the effective diameter was much less than original. I replaced some lines, did some by-pass surgery here and there, made some headers a loop design, and replaced some SSs to comply with ANSI Z 358.1 2004. Now, we have ANSI Z 358.1 2009 in U.S. Is there a national/local/company standard where you are? My advice is do not install a new system on the minimum edge of the standard/design. You show only 1 SS. What if "they" want to add another later? Our company standard said we have to design for 2 SSs flowing on a multi SS system.

I am not suggesting your design is not adequate. It may be perfectly fine. Just know your standard, if there is one. Think about the "what ifs" that may happen. Be sure your pump can take a larger impeller and motor and your piping can take higher pressure, if needed in the future. I think most SS heads work from 15-90 psig (1-6 barg).

Good luck,
Latexman

 

[Mechh007]

Latexman,
Thanks for our valuable response. We have increased the suction pressure from 30 Psi to 50 Psi. SO we can have discharge pressure 50+52.2 = 102.2(7.04 bar). Losses is 4.87 bar, So we can have 7.04 - 4.87 = 2.1 bar. It is ok know

 

[Latexman]

Excellent!

Good luck,
Latexman