CLOSED LOOP PUMPING 1

[Rocky385]

HI EVERYONE,
I HAVE A SYSTEM WHICH IS A CLOSED LOOP CHILLED WATER SYSTEM WITH A 7.1 BAR PUMP. IT IS SUPPLYING WATER TO A CIRCUIT WHICH HAS 80 METER VERTICAL LIFT. CAN ANYONE TELL ME WHAT WILL BE THW PRESSURE JUST AT THE BOTTOM OF THE LIFT? I KNOW FOR CLOSED LOOP SYSTEM THE PRESSURE INSIDE THE SYSTEM WILL BE THE PUMP PRESSURE AND WE DONT NEED TO CONSIDER THE STATIC HEAD. BUT I WANT TO KNOW THE PRESSURE JUST BELOW THE VERTICAL LIFT.. WILL IT BE 7.1 + 8 = 15.1 BAR? BECAUSE MY PIPE CAN WITHSTANT ONLY 10 BAR...CAN ANYONE PLASE HELP???

 

[trashcanman]

First, please do not post in all Caps. It is like you are shouting (like a Vogon).

The internal pressure in the pipe is given by Bernoulli's' equation, which is the pump head plus elevation head. Better select your pipe, fittings and valves to take the working pressure + 1.5 x working pressure.

 

[bimr]

The term pump head however, has nothing to do with height. It is a term that is used that is the pumps ability to overcome the friction that is created when fluid flows through a pipe. Once a system is filled with fluid – the pump merely has to overcome this friction. An easy way to think about it is that as water goes up one side of the loop, it also comes down the other side of the loop. The two cancel each other out.

A good illustration is like a Ferris wheel. One car going up balances the other car coming down. The cars are in balance and the motor only has to overcome the friction in the bearings and we are off and spinning. The pump in a closed loop system in simple terms is like the motor on a Ferris wheel. All the pump has to do is overcome the friction that is created when the water rubs against the pipes – and that friction has nothing to do with height. In fact we could take a ten story building, size a pump for it, knock the building over on its side and we would still need the same exact pump. Pump head has to do with the friction caused by flow, that is all. We have to have a pump with enough head to move the fluid through the longest loop.

In a closed loop system, like in a chilled water circuit the height of the building is irrelevant because the work done(Pressure) by the pump in lifting the water column is gained when the water column falls by the same height. Hence consider only the dynamic pressure head only, i.e the pressure that the pump has to develop to circulate the fluid overcoming the friction loss and fitting pressure loss. Also add the pressure loss in the most remote AHU or FCU. That will give the Head required by the Pump.

 

[BigInch]

If you basically only had a high lift and a very large diameter pipe (friction negligible) in an open system, or a partially filled closed loop system, where you had no to little water in the downcomer, I'd say that pump head would have everything to do with height.

 

[LittleInch]

Rocky 385.

It depends a lot on whet the pump is located relevant to your lift.

However even in a static situation, the pressure at the lowest point will be approx 8baag. Add on whatever static pressure is present at the highest point as this is rarely exactly 0 barg and you're getting close to your 10 barg pipe limit.

Foot a flowing condition, there is likely to be some additional pressure due to friction, but how much depends on where the lowest point is in the closed loop cycle. Close to the start it will be high, close to the end much lower.

Draw a sketch of your circuit and we can make some judgement.

Bimr is correct in terms of working out the pump head losses, butthe height of the building will impact the actual pressure seen at the lowest point.

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

 

[bimr]

The piping system at the bottom will have 7.8 Bar of pressure with the pump off.

If the pump is on the roof, when the pump turns on, the pressure at the bottom will increase from 7.8 Bar to 7.8 Bar + 8 Bar - piping system losses on the way down. Note that if the pump is on the roof, you will need an additional 0.3 Bar to avoid pump NPSH problems.

If the pump is at grade, when the pump turns on, the pressure at the pump discharge will increase from 7.8 Bar to 7.8 Bar + 8 Bar. The pump suction pressure will be 7.8 Bar + 8 Bar - System head losses.

 

[Rocky385]

Thanks for the information. I will try to upload the drawing of the circuit. I will mention the position where I have to calculate the pressure

 

[LittleInch]

Your drawing is a little odd as it seems to show only 40m vertical height.

Assuming it is only 40m, the pressure would seem to be less than 8 barg, BUT, we don't know what pressure exists at the high point when the pump isn't running, i.e. what the pressurisation system /expansion tank is. There will be one.

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

 

[Rocky385]

Very sorry about the drawing...the height is 80 mtrs..but when the pump is working what will be the pressure at the base of the vertical riser?..I know for the whole system the static pressure cancel out but here in this point where the gauge is connected what will be the reading is it the 7.1 bar or 7.1 + 8 bar?

 

[bimr]

Assuming you have no additives that would change the specific gravity of the water, the minimum pressure at the bottom of the system with the pump off, and the system "cold", would be 7.8 Bar (80 meters).

If the pump is on the roof, when the pump turns on, the pressure at the bottom will increase from 7.8 Bar to 7.8 Bar + 8 Bar - piping system losses on the way down. Note that if the pump is on the roof, you will need an additional 0.3 Bar to avoid pump NPSH problems.

If the pump is at grade, when the pump turns on, the pressure at the pump discharge will increase from 7.8 Bar to 7.8 Bar + 8 Bar (pump system head losses). The pump suction pressure will be 7.8 Bar + 8 Bar - System head losses.

 

[LittleInch]

You still haven't said anything about what the pressurisation of the loop is. All closed systems have some sort of expansion/contraction tank often kept at some sort of over pressure. This could be 0.5barg or 5 barg. Makes a big difference to what your pressure might be.

Your system diagram is very basic and not dimensioned. For example, is there any form of flow control and is it up steam our downstream of your lowest point.

Therefore it is not possible for anyone to say exactly what the pressure is at your lowest omit.

As bimr says, the pressure will be 7.8 barg + pressurisation pressure + 7.1 bar minus frictional losses.

It does look like your 10barg pipe is in danger of seeing pressure higher than 10 barg, but by how much I can't judge from the data provided.

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