18 replies to this topic

[processengeng]

I have a centrifugal pump and the discharge line will be split into two streams. For each stream, there is a mass flow meter and flow control valve to control the flow rate. No other control for this system. Any one can advise if it's ok to put two control valves in parallel?

Based on my previous project experience, usually I will set free for one of the streams ( no control valve) if the pump discharge is split into a few destinations.

Thank you in advance.

[latexman]

Do you have a P&ID we can look at?

[fallah]

 

Hi,

 

You can use parallel FCV's along two parallel pump discharge streams...the pump will adjust itself with the downstream conditions otherwise the total discharge flow to be out of acceptable operating range of the pump...
 

[emma0618]

but i only have one pump going to two destinations. any problem if one control valve for each stream?

[fallah]

but i only have one pump going to two destinations. any problem if one control valve for each stream?

 

Even without control valve at each stream pump discharge will divided to two streams going to two destinations...what's the problem?
 

[cea]

This system can work, provided pump hydraulic is done considering all possible scenarios.
 
You have not mentioned destination pressure requirement & elevation for both destination. Are those same?
Note that with your system, there will be shift of system curve viz-a-viz pump curve when you put control valves on both lines. If intersecting point of these curve shift from duty point for required scenerios, you will observe continuous movement of control valve & it will never reach equilibrium.
 
Your statement "Based on my previous project experience, usually I will set free for one of the streams ( no control valve) if the pump discharge is split into a few destinations. " is correct & is standard practice.

If your total flow is expected to remain same then there can be one flow meter & control valve controlling main flow & other flowmeter (on main pump discharge) with control valve on other line will ensure total flow. This arrangement can definitely work well.

[processengeng]

Hi fallah and cea, thank you for ur reply. my problem now is I am not sure if these two control valve will conflict with each other?

This two streams are sent to two different reactors and the flow rates are different. Is it because that the reaction in the reactors requires accurate feed in to ensure the product quality? These two reactors are located at the same elevation and the destination pressure are the same too.

[fallah]

Hi fallah and cea, thank you for ur reply. my problem now is I am not sure if these two control valve will conflict with each other?

This two streams are sent to two different reactors and the flow rates are different. Is it because that the reaction in the reactors requires accurate feed in to ensure the product quality? These two reactors are located at the same elevation and the destination pressure are the same too.

 

Hi,

 

As i mentioned in my first post you can use two parallel CV's in parallel lines taken off from pump's discharge line without any conflict same as what might be happenef in series CV's. The problem may be happened if total discharge flow will be out of allowable operating range of the pump; either lower or higher...
 

[S.AHMAD]

In agreement with Fallah as long as you size the control valve correctly..You can even installed three or more valves in parallel as long as the pump has sufficint capacity to cope with the system demand.

 

s.ahmad

[latexman]

How will you handle the pump's minimum flow requirement?

[fallah]

How will you handle the pump's minimum flow requirement?

 

latexman,

 

It will be handled with a flow measuring element at main discharge line and a control valve on take off line from this line return back to the suction drum..in any case if the flow in main discharge line would be lower than the specified minimum flow, the control valve will be opened to compensate the flow shortage such that the total flow in main discharge line to be at least equal to minimum flow...

 

As we can see there is no difference in pump's minimum flow handling between one discharge line and two (or more) parallel discharge lines taken off from the main discharge line...

[latexman]

fallah,

 

That is not the mental picture I got based on post # 1 and 7.  The "main discharge line" was introduced by cea, not processengeng.  I admit I could be wrong, but let us let processengeng answer questions on his process.

[fallah]

 

- Two pumps (one in operation, one stand by)

- one FCV on each stream (flow rate to each destination is different)

- Minimum flow line with RO

 

Please let me know if you have concerns on the configuration.

 

processengeng,

 

There is no specific concern on the configuration you uploaded...
 

[latexman]

Agreed, no concern.  My question was answered.

[Padmakar Katre]

I have a centrifugal pump and the discharge line will be split into two streams. For each stream, there is a mass flow meter and flow control valve to control the flow rate. No other control for this system. Any one can advise if it's ok to put two control valves in parallel?

Based on my previous project experience, usually I will set free for one of the streams ( no control valve) if the pump discharge is split into a few destinations.

Thank you in advance.

 

Hi,

You can certainly have two parallel FVs subject to,

1. One of the FIC is cascaded with LIC of source vessel - inthis case flow may not be constant

2. Third control valve in minimum flow with low level override to safeguard pump from running dry or loose suction

[Mahdi1980]

Hi

As friends told you, there is no conflict between two control valves on parallel streams. However, there might be some pressure fluctuations in the system since, as one control valve much more open or close to regulate the flow rate of its stream the upstream pressure will change according to the principals of centrifugal pump. So, as the upstream pressure of another stream is equal to this one, it has an effect on the entire line pressure.

To reduce this effect the kick back line to operate.

 

Mahdi

[S.AHMAD]

Dear Mahdi...

my mind told me that fluctuation occurs even if we have only 1 valve..... however once the system reaches equilibrium, the fluctuation shall e minimized...

 

 

Salam from

S.Ahmad

[processengeng]

Anything I need to take note if I want to size the control valves? The normal flow rate through the control valve is different..thanks

[danyelson]

In case two control valves will be considered for two discharge lines coming from a common pump (the destinations are critical and you need a flow control for both lines), is very important to define the minimum flow. Also, is very important the process control (one valve related to a cascade), taking into consideration that there is a fluctuation between OP and SP, and the system disturbance is increasing. Otherwise, if only one destination is critical, you could leave the other line free.