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 the header is common for all pumps irrespective of their flow/head capabilities when operating as a single unit.

 

[LittleInch]

See below:

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?

At the moment that's all you have to play with.

• 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.

It's fairly big, but huge is 5000 psi down to 300 like you get at well chokes. The pump would be oversized, not over designed

• 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?

No. You say the head of pump A is 150 psi (sounds like a conservative guess by the Process engineer to me), but it's a lot higher than 62 psi...

• 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?

You're over complicating it.

• What recommendations would you give me to address this problem? Is there any other thing I should pay attention to?

First understand the whole process in this section, understand max and min flows and pressures. E.g. what it the shut in pressure of pump B. What is the design pressure of the header? what is the design pressure of wherever this 102 line ends up? Can either of the pumps create excess pressure in a shut in condition... What is the consequence down stream of either pump tripping and you get 100% of fluid A or B on the downstream units

• Any references do you recommend for such pumping system cases and for control valve pressure differential determination?

This is a simple system whereby one line (pump A) is essentially injecting fluid into a line flowing by pump B

• Any general tips and references are appreciated.

Talk to your process engineer and get them to explain the system and the limitations of it. And ask them where did 150 psi come from. or 62 psi for that matter once you add in more flow from pump A. Expect no clear answers on that one...

 

[sweetliver]

See below:

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?

At the moment that's all you have to play with.

• 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.

It's fairly big, but huge is 5000 psi down to 300 like you get at well chokes. The pump would be oversized, not over designed

• 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?

No. You say the head of pump A is 150 psi (sounds like a conservative guess by the Process engineer to me), but it's a lot higher than 62 psi...

• 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?

You're over complicating it.

• What recommendations would you give me to address this problem? Is there any other thing I should pay attention to?

First understand the whole process in this section, understand max and min flows and pressures. E.g. what it the shut in pressure of pump B. What is the design pressure of the header? what is the design pressure of wherever this 102 line ends up? Can either of the pumps create excess pressure in a shut in condition... What is the consequence down stream of either pump tripping and you get 100% of fluid A or B on the downstream units

• Any references do you recommend for such pumping system cases and for control valve pressure differential determination?

This is a simple system whereby one line (pump A) is essentially injecting fluid into a line flowing by pump B

• Any general tips and references are appreciated.

Talk to your process engineer and get them to explain the system and the limitations of it. And ask them where did 150 psi come from. or 62 psi for that matter once you add in more flow from pump A. Expect no clear answers on that one...

Thank you very much for your valuable answers and the recommendations you have mentioned. I will not complicate this problem and resume with the calculation I have listed in my first point.

The recommendations you have mentioned are crucial and understanding the system we aim to change is very importance to have the desired outcome. I have made my best to understand the system but I also have to address other projects (I have over 4 projects of different technical nature - It raises concerns about the deliverables quality)

Pump B is willing to provide higher pressures but I think its pressure gets dropped by a flow control valve for a certain flowrate.

Thank you again.

 

[sweetliver]

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 the header is common for all pumps irrespective of their flow/head capabilities when operating as a single unit.

Thank you for your reply. I think I have made a mistake in not explaining that this header routes the Product A/B mixture to a storage tank. It is a pipe with a certain pressure of 62 psig in the beginning of the 10" pipe (according to the hydraulic study recorded I have been provided with).

I will refer to the process engineer to seek more explanation about their 62 psig assumption.

Thank you again.

 

[sweetliver]

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

Thank you. I believe I will provide more details and possibly schematics in the future. This will make the question put in a better shape.

Thank you again.

 

[LittleInch]

A 10" line with 62 psi going into a storage tank seems quite high unless its a long distance or you're going up a hill... Or there is a further control valve downstream the point where the flows join?

It's not easy to comment further without seeing the whole process diagram from end to end and what the shut in pressures are for each pump.