11 replies to this topic

[vinod]

Hello Sirs
I have a closed loop cooling system,in which i am using cooling water to remove heat from a nitrogen plant and then cool the cooling water in a plate and frame heat exchanger using sea water.Cooling water is recirculated at 600 m3/hr.It enters the exchanger at 49 deg C and leaves at 39 deg C.

My question are:

1)Whether i can connect the suction of the pump to the exchanger withhout having a holding tank in between.
2)Is is a rule that i need to maintain the pressure of the cooling water side higher than the sea water side ,so that even in case of a leak,sea water does not enter the cooling water circuit.

Thanks a lot

Vinod

[fallah]

1-Normally no; the holding tank is necessary because of supplying the NPSHa for the pump and adding the make up water.

2-Yes,the pressure of the sea water side should be lower than the cooling water side.

[rsk]

You have a intermediate closed loop for heat removal from nitrogen plant to sea. You can connect pump at outlet of heat exchanger but holding tank is must to provide make up for losses, for accomodating swell & shrinkages even though pump NPSH may not be a problem. So i have not ever come across closed loop without expansion tank. you can install expansion tank riding at suction of pump or discharge of pump, both have their unique advantages. tank riding at discharge limits the discharge pressure from pump theirby protecting downstream equipment or meeting the process requirement. tank at suction of pump provides NPSH, & increases the pressure profile through out the system increasing the pressure rating of whole system.

[vinod]

Sirs

Thanks a lot for the reply.

I am having a potable water distribution pump from which i am taking a line to provide make up water in the suction of the closed loop recirculation pump.This is a manual operation since the losses due to leak etc will be minimal in a closed loop.And also if if the NPSHa for the pump is almost 4 m higher than the NPSHr ,do i still need a holding tank.

Thanks a lot

Vinod

[Art Montemayor]

Vinod:

When you don't supply detailed information or basic data, you can't expect the responses to be detailed and complete to your satisfaction. The result is that you continue to return with follow-up responses or further explanations that simply continue to add on to the thread with not much cohesive detail. Both fallah and rsk are giving valid and useful advice - but they can't furnish the details required to fully satisfy what you are asking or seeking if you don't make YOUR own contribution and effort to your own cause.

You fail to supply the forum with a detailed sketch of your proposed system and, consequently, leave out the details of what specific NPSHa you are proposing, the need to supply a reservoir, and the need to supply an expansion tank within a closed, circulation system. In any closed loop circulation system you must furnish a means by which you monitor the fluid inventory within the system (usually as a liquid level in a reservoir) in order to be able to pump in or inject make up fluid. An expansion tank is required in the event you are carrying out heat transfer with the circulated fluid (which is something you also fail to identify). It is obvious that the recirculated fluid is being used as a heat transfer medium in your system - or it is a seal fluid (which would also accumulate heat and require an expansion tank).

The main point I want to bring out here is that you should furnish your "part of the bargain" in obtaining free engineering counsel and advice: furnish a detailed sketch and all of the basic data. That way you will obtain the best, accurate, engineering comments instead of guesses, generalities, and speculations.

[vinod]

Sorry sirs.

Please find the attached sketch:

1) So, do I need to still consider an expansion vessel, as my cooling water leaves the exchanger at 39 deg C and enters the exchanger at 49 deg C? So, after the exchanger I have a volume contraction.

Regards

Vinod

Attached Files

•  SKMBT_C45010061321500.pdf   132.79KB   152 downloads

[fallah]

Your arrangement isn't proper for a closed loop pumping system.

Anyway, if you insist on not considering any expansion tank, you have to consider a relative high pressure source of make up water with a PCV (back pressure regulator) followed by a check valve in the connection point of make up water line to pump suction line. PCV would maintain the suction pressure value corresponding to the NPSHA needed for pump protection. Also, you need to consider air valve(s) in high points of the closed loop in order to release any trapped vapor/air pocket.

[vinod]

Thanks Fallah

Why would i need a PCV to manitain the pressure in the suction of the loop pump?

Regarding putting a check valve in the make up water dosing point-----yes i will include it.Thanks.

And regarding the air release valve,will it be like a PSV which will pop and release the accumulated air/vapour/gases collected in the high points.

How can i start the calculation of NPSHa for this system,since there is no difference in the pressure head,no static head,no velcoity head since it is the flowrate ,and only factor is the friction factors .So does it mean that my pump will run and keep increasing the discharge pressure since the differential head remains the same due to smae flowrate-----i think i have got confused and tried to confuse everyone a lot------

[fallah]

NPSHa would be NPSHr of the pump plus a margin (almost 1 m), and corresponding pressure of this value could be set point of PCV downstream.

Air valve is a valve equipped with a floater moves downward in the case of air trapped and air to be released through a hole being opened due to floater falling.

It is reminded again: this configuration wouldn't be a desirable one and may have some unforeseen operational problems.

[chemsac2]

Vinod,

Firstly answers to your two questions:

1. An expansion tank would surely be needed even if it is a case of contraction. Reason is simple. Assume water density at 39 deg C to be 1000 kg/m3 and system volume is 10 m3. This means System mass is 10000 kg. Now if temperature increases to 49 deg C, density would decrease to 980 kg/m3. This would mean that system can contain only 9800 kg without pressurising the system. Hence, to avoid pressurisation, 200 kg of mass needs to be removed from system. This works even when it is in contraction, but other way round i.e. pressure would decrease. You can never be sure of temperatures in your system when specific temperature control scheme is not present and hence always recommended to have expansion vessel. As name recommends, main purpose is to provide expansion, NPSHA benefit is secondary. In any case NPSHA would not be a big problem for potable water pumps.

2. Since circulating system is potable water, it would be must to maintain potable water pressure above sea water pressure.

As for question of NPSHA, I think it would be based on pump vendor requirement. If pump vendor requires 3 mlc as NPSHR, procedure would be as follows:
1. Decide location of expansion vessel to ensure NPSHA at pump suction flange as 3 + 1 mlc margin.
2. Calculate pump suction pressure based on expansion vessel elevation and pressure (expansion vessel pressure can be maintained at slight positive pressure by gas blanketing)
3. Now pump differential head would only consist of frictional and equipment/instrument losses from pump discharge to pump suction. Changes in static head would be considered negligible as density change from 39 to 49 deg C would be small.

Hope it helps.

Regards,

Sachin

[vinod]

Thanks Sachin and Fallah

Now i have decided to go with an expansion vessel connected to the suction of the pump.But since no blanketing is available i would go for an elevated expasnion vessel but opne to atmosphere.How would i size the expansion vessel.I was planning to base it on 0.5 % of recirculation rate ,which is a literature based value for small leakage in closed loop multiplied by 2 to 3 hours.This expansion vessel will have a level switch ,which will activate the addition of make up water into it.

Is my understanding correct.?

Shoudl i locate my expansion vessel above the highest point inthe circuit.?

Thanks a lot

Vinod

[fallah]

Glad for your proper decision!

The attached will help you out in the right way.

Attached Files

•  ASHRAE Journal - Understanding Expansion Tanks-Taylor Proof.pdf   4.31MB   103 downloads

Edited by Art Montemayor, 16 June 2010 - 02:31 AM.