Pumps in Series, Different Size

[ProcessHVAC]

We are commissioning a small train fuelling facility and we are having some issues with a train fuelling nozzle. Apparently, the manufacturer claims it is activated by vacuum and then pumps dead head pressure seals it, but we are finding it doesn't shutoff some times. The contractor claims the system needs a bit more pressure. The currently installed pumping system is 160 USGPM, 35 psi. Two pumps, duty and standby (Pump 1 and Pump 2). The system was started and works well, we tested the pumps and they operate as intended and we are getting the flow we wanted. I am thinking about using a series system to boost up the pressure. I have done pumps in series for HVAC applications but they are usually small pumps so I am trying to figure out actual operational details of putting these pumps in series.

Assuming the system needs a bit more pressure, my options are:

A) Increase the size of the impeller and boost up the pressure to around 47 psi, I am not sure if this is enough though.

B) Put Pump 1 and Pump 2 in series and boos up the pressure to around 70 psi, we have control valves for throttling so excessive pressure isn't a problem. Only problem is I lose standby pump.

C) There is another set of pumps (Pump 3 and Pump 4) they are the same model but with a smaller impeller (160 USGPM 22 psi) used to transfer fuel from tanker to below grade tanks. I am thinking about putting the two systems in series.

I would like to pursue alternative C, as it allows me to keep standby feature. The question is, I have heard that putting pumps of different size in series can be problematic, how can I overcome this?. Flow control and pressure is not critical as the system almost self-throttles itself at the nozzle and as I said we have globe and modulating ball valve in the system. I have a main plc doing pump start stop, etc and can reprogram it.

 

[EnergyProfessional]

here some thoughts:
- determine exactly if you need redudnancy or not (or have your client determine for you and sign off)
- determine exact flow and pressure needed. The "around 47 psi, I am not sure if this is enough though" is a big red flag to me. I'm not sure if 47 psi is enough either, but I'm not the designer
- if possible, use one pump. Only if none with high enerough pressure is available, use series.
- look at B&G and other publicatiosn on the issue, it can be done, but in general you always want series/parallel pumps to be same size. Contact pump sales rep, they can help. Pump manufacturers also have online tools to tell you that.

 

[ProcessHVAC]

Sorry, I actually wanted to post this in the Pump Eng forum, wonder if I can move it ?

 

[Artisi]

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