9 replies to this topic

[RachelZ]

Hi all,

 

I'm working on a project and have managed to royally confuse myself.

 

I'm resizing a pump for a cooling water system. The cooling water passes through a control valve that is currently calibrated for a maximum flow of 200 gpm. Can I increase the flow through the valve to 800 gpm without completely replacing the valve? It seems to me that the head from the pump should be able to force that much through but I'm really not sure.

 

Thanks.

[latexman]

Maybe, a fluid flow analysis of the cooling water system is needed to know for sure.

[chemproc]

Can you show some pictures? Also, If the maximum flow through the valve is 200 gpm then forcing 800 gpm through it might not be such a good idea

[RachelZ]

I really was just wondering if it was possible it all. It sounds like from what you guys have said that it IS possible but maybe not a good engineering decision. Thanks!

[meetyourmaker]

increasing the flow through the existing valve by a factor of 4 willl require an increase in pressure drop through the valve by a factor of 4^2. I dont think this valve will work. A very large pump will be required and valve upstream piping design pressures will have to be checked

[latexman]

It really depends on what you have.  I've seen 3" to 6" control valves that have 2-4 choices of trim.  Each step up has a Cv about 1.5X the previous one.  If you have a "big' control valve for your service with a "small" trim, you might have a shot.  This is the kind of thing that would be done if one used multi-generational planning,  Realistically though, the chances of a trim change and getting 4X the flow is pretty small.  Thus, the need for a fluid flow analysis.

[breizh]

As stated by others fluid flow analysis is required , a sketch and a pump curve will be of great help !

 

 

Breizh 

[adewumi_bj]

If the pump can produce your desired new capacity without going out of it's efficiency range I will suggest you change your existing valve plug to a quick opening type or find a way of increasing your pump's suction velocity head.

[Atttyub194]

Dear RachelZ

 

Good day!

 

Lets first come to the conclusion first , my first answer is probably no.

 

Now let us describe, why.

1. While specifying a valve at pump for cooling water system, we normally use minimum opening of 15-25 %  depending on valve design and maximum opening of 75-80 % . Beyond this range, increase in valve opening will not effect increase in flow and below this range you may observe severe pitting and damage to valve due to cavitation
2. Specifying a valve for cooling water service our choice is always a cheapest valve and also there is always a limit to pressure drop and such valves are limited by critical pressure drop and metallurgy of the plugs and internals
3. It is not important at what range you have calibrated the valve but the most important aspect is at what percentage opening you are operating the valve and how far you are from 100 % opening
4. In case of cooling water , the increase in flow in only possible by reducing pressure drop across valve
5. My observations can be confirmed from supplier submitted graph of CV versus pressure drop for this specific valve
6. Even incase you are able to increase the flow by increasing pressure drop please do check the economics due to increase in power versus cost of valve
7. I will be in better position to comment if you can share the specification of the valve and the scheme

 

Best regards and God Bless You

[S.AHMAD]

My view are as below:

1. If you are using existing pump, most probably the answer is a big NO,

2. On the other hand, if you are selecting and sizing a new pump, then the answer is possible.

3. Let me explain:

the basic flow valve equation is:

Q = Cv x SQRT(DP/SG)

If we are using the same valve, that means Cv is equal and for the same liquid then SG is also equal. Using this equation then you can determine the additional pump head required to deliver the volume:

DPnew = DPold x Square(800/200).

I hope you get what I mean....