Two Pumps of Different Product and Ratings Pumping into a Common Header

[sweetliver]

Hello,

I am handling a pump capacity upgrade project in which I have done a comprehensive overview of the system components. The pump (Pump A) I am going to replace is going to pump a liquid in a 4" line with a flow rate that is 4 times less than the flow rate of another product pumped by another pump (Pump B - of different capacity and different head) in an 8" line. Both liquids are going to mix in a 10" header according to a new pipe routing that we are going to introduce. According to an engineering record of the hydraulic study I have for that 10" line header (not yet commissioned - the project request I have is sudden. The 10" line was constructed base on another project input), the pressure in the beginning of the header (where the products are going to enter) is 62 psig. I do not know what the basis is for this 62 psig as the process specification I have been provided by the process engineer is that the upgraded pump (Pump A) should have a discharge of 150 psig. I have reached a stage in this work in which I have to provide the required pressure differential across the control valves to the process control engineer for the control valve purchase.

My questions are the following:

• For the control valve differential pressure, should I use the 62 psig as the input for my pressure drop (head loss) calculation, by assuming the the downstream pressure end is 62 psig and add back the head loss up to the control valve outlet?
• Is not a drop from of 150 psig to 62 psig huge? I am sure that the distance from the pump discharge to the control valve inlet would not contribute in much head losses. This gives me an impression that the pump will be overdesigned in terms of impeller size, as the flowrate can be achieved with a smaller pump, instead of throttling the generated head in the control valve.
• The pump (Pump B) of the product in the 8" line has a lower discharge pressure (its pressure is throttled to around 60 psig in the existing system) but higher flow rate capacity. Is it going to enter into the 10" header and overcome the NRV and the pressure of Pump A if Pump A liquid pressure is not dropped across the control valve to up to 62 psig?
• It is the first time I handle a system with two pumps of different ratings that will be routed to pump in a shared header. Is it a sophisticated case, or am I overcomplicating it?
• What recommendations would you give me to address this problem? Is there any other thing I should pay attention to?
• Any references do you recommend for such pumping system cases and for control valve pressure differential determination?
• Any general tips and references are appreciated.

Thank you.

 

[Doug Hunter]

I recommend that you add a hydraulic schematic to go with your verbal description.

 

[Artisi]

You're over complicating the problem, just treat each pump as an individual unit and calculate the operating condition for each unit.
Start from the inlet to each pump to the discharge point into header that already has a pressure of 62 psi. That means the difference in elevation from inlet to discharge, the total friction losses in the inlet and outlet pipework including valves etc. plus 62 psi, do this at various flowrates to develop a system curve for each pump.
This will give you flowrate for each pump - - - however that assumes the head required at the pumps combined flowrate is 62 psi. You need to establish where / how / why the header pressure is 62 psi.
Trust this helps with understanding your problem.
You can run as many pumps as you want to a header, but bear in mind the pressure at this point is common for all pumps irrespective of their flow/head capabilities when operating as a single unit.