Close loop charging 3

[saeedplc]

Hi dear experts,

Dears i am very new to mechanic knowledge and my below question maybe seem very simple to answer but are very tough for me to get.

Could you please let me know what pressure amount shold a close secondary loop of a chilled water be filled to? For instance a loop can be charged up to 2 or 3 bar or everything. What determine the right pressure?
Should the loop be precharged with water when the valves of the equipments are open or close or in working condition?

designer of the system has mentioned duty point of the pump as 130m3 1.7bar.
The duty point is when the equipments are in their full flow or their working conditions?

Regards,

 

[1503-44]

You need to charge enough for the pump to be able to lift the fluid from the level in the suction to the level of the high point, plus the pipe friction loss. Pump head must be > high point elevation + pipe friction loss - suction level.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

Mr 44 - For a closed loop system the pump head is only the frictional losses around the loop from outlet to inlet no? level only gives you the actual pressure in the pipe that the pump need sot be capable of withstanding from a pressure containment point of view.

However the other answer is correct. The pressure ( define where in the system you are measuring it) needs to be sufficient to maintain at least say 1 bar at the highest point in the secondary loop. Usually there are expansion tanks required also.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Only if the loop is completely filled with fluid, i.e. "fully charged". If you dont charge to at least vapour pressure at the highest point, or drill a hole before the downcomer and let the fluid waterfall down the return pipe, as you know, you can get vapour, or air in the case of a hole, in the downcomer and a low level suction head. It would look like the hydraulic profile of a cooling tower.

The problem can be, if the pump only has enough head for friction drop, what do you do when you have to drain the loop and your pump cannot refill the loop to the high point. You have to find some other way to refill it. It's the equivalent of designing a pipeline to go over a big hill, but only provideing enough pumping head for outlet - inlet level - friction. You'd have to prime the pipeline every time you drained the download after forgetting to close the discharge valve.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

I think we need to assume it's a filled system if the OP is asking what pressure it needs to be at.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

That depends on where his pressure gage is. If it's not at the high point of the system, your answer may not be correct. Although you correctly specified his gage needs to be at the top, he did not. He could be looking at a pressure gage near the pump discharge flange.

And as we have seen before, no need for expansion tanks if you run partially filled with a cascade, nor is there an actual need to charge higher than vapour pressure, if you can and want to do it that way. Not my idea of the best way, but it works and my answer covered either way he wanted to do it,

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[saeedplc]

Thank you so much for all your replies.
Could you please answer all my questions?

I need to say the guages are installed on the suction and discharge sides of the secondary pump.
For charging the loop a pump is used that can increase pressure upto 5 bar.
If anything is needed to know for answering please let me know.

Regards,

 

[LittleInch]

Yes, you need to know what is the height between the guage and the highest point of the secondary system.

The loop should be full of water at all times

Duty point of the pumps should be max flow.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Assuming 1.7 bars is your pressure loss of the entire loop at maximum flow rate, a loop completely full pipe of water at room temperature, the pressure reading at pump discharge (barg) should be =
(highest point pipe elevation m - gage elevation m + gage height over pump centerline m) / 10.34m + 1.7 barg.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[saeedplc]

Thank you

Is it possible to meassure the pump head pressure( pressure across the pump) and then find the flow by refering to the pump curve?

 

[EdStainless]

Maybe, is your pump curve an actual test curve for your pump or is it the guaranteed curve?
You can also measure motor power (real power) to compare.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[saeedplc]

I mean the curve that the pump's manufacturer offer.

 

[LittleInch]

Saeed,

Together with electrical power it could give you a flow somewhere around +/- 20% accuracy.

If that's good enough for you then give it a go. Also depends on the accuracy of the pressure readings.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[saeedplc]

@Littleinch thank you for the reply

The pump is working with 40HZ through vfd and the pump head is 1.7 bar and the pump is a circulating pump.
Now i want to get the flow value because there is no flowmeter.
It would be appreciated to help me using pump affinity law to get the flow.

 

[1503-44]

Take the discharge pressure - suction pressure and look at the corresponding flow on the pump curve.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[saeedplc]

Thank you for the reply

pump curve offered from the manufacturers are usually at frequency 50 or 60 HZ but the pump is running at 40HZ!

 

[LittleInch]

Then use the affinity laws

flow is proportional to the ratio of speeds, head is proportional to the ratio of speeds squared.

So in your case, for any one point on the 50 htz curve, reduce the flow by 4/5 and reduce the differential head by 0.64 (0.8 * 0.8)

Then you can create your own curve at 40 htz.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

That's it.

Sorry. I thought you had a curve for 40hz. Some mfgrs show multiple speed curves, some don't and others show only the 60. Others only 50.

I must polish that crystal ball.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[saeedplc]

1503-44 (Petroleum)25 Feb 22 16:07
Assuming 1.7 bars is your pressure loss of the entire loop at maximum flow rate, a loop completely full pipe of water at room temperature, the pressure reading at pump discharge (barg) should be =
(highest point pipe elevation m - gage elevation m + gage height over pump centerline m) / 10.34m + 1.7 barg.

For the formula mentioned is not it important where the pump is located according to the height?
Suppose the pipe over the pump is 5 meter and the heigh from lowest part of the loop to the suction of pump is 15 meter then the loop should be charged to 1.5 bar or 2 bar or what?
It is a little vague for me. It seems total height and guage location should be considered.

 

[1503-44]

Pump centerline elevation is taken as zero. All elevations are + if above and - if below.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.