Flow stall after pipe drops down 9

[markboc]

Hello,

we have the following problem: A heat exchanger (15 m) is fed by a pump (0 m). It is a pipe network where other heat exchangers on parallel pipes are located much lower. The pressure gauges read low to negative pressures right before and behind the heat exchanger. Throttling behind the heat exchanger seems to solve the problem. It is suspected that the flow stalls after the heighest point of that branch of the piping system. Nonetheless the flow rate through that branch is higher than through the branch that contains the other heat exchangers, located lower.

1) Can anyone direct me to literature or suitable key words for google, to get more information on the problem of the flow stalling? We suspected because the flow experiences a free fall and accelerates the vacuum in the heat exchanger is created.

2) Another mitigation which was thought of: install a throttling valve behind the pump before the pipes branches in order to increase the pressure. The increased pressure should make sure that the 15 m heat exchanger is supplied with medium. My questions here are:
2.a) The pump head matches the pressure losses of the pipe network. If I install a throttling valve the pressure will rise, but only before the valve. The increase in pressure should match the pressure drop across the valve. So in my opinion it is not possible to control the pressure in the pipe network with this method.
2.b) I am correct in assessing that the pressure at branching point does need to be greather than 15 m + pressure losses across the pipe + pressure loss across the heat exchanger. I suspect this does not pose a problem as long as the pressure loss across the other path is high enough. This would lead me to think we can ensure proper operation by throttling in the branch where the 15 m heat exchanger is NOT located.

Basically I'm trying to determine at which points we can try to control the flow in a manner, where we have no negative presssure at 15 m.

I'd appreciate your input on the situation,

have a nice weekend!

 

[katmar]

The purpose of the vent is not to release accumulated air. The vent is there to break the vacuum that forms at the top of the downleg by controlling the pressure there to 0 barg. This is a continuous duty and cannot be performed by occasionally opening a manual valve.

The real problem that the OP was experiencing was mechanical damage to HX1. This is most probably caused by boiling of the water (because of the vacuum) in the downleg resulting in fluctuating pressures and cavitation.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[markboc]

Thank you all again for your input.

We made the decision on the design and will start ordering soon.

As per 1503's simulation we felt comfortable of installing control valve in the Hx1 downleg. This should solve the pressure problem and help reduce flow over Hx1.
We will also install a FCV d/s of the pump as originally planned.

I will keep you posted after installation later this year on how this turned out.

Take care!

 

[katmar]

As a retrofit operation, using a control valve is probably easier to implement than the high point vent option. Installing the significantly larger diameter piping required by the vent into existing buildings can be difficult. So I am with you on this decision.

I strongly recommend that you use my suggestion of 14 April (and confirmed by 1503-44 on 16 April) and install the valve near grade level (i.e. junction with Branch 3) and use it as a pressure control valve to prevent the development of vacuum conditions immediately downstream of Hx1. You should leave V1 in position to control the flow rate through Hx1 because fixing the downstream pressure is unlikely to guarantee the correct flow through Hx1.

Thank you for coming back to inform us of your decision. I look forward to the final wrap-up of performance results.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[1503-44]

I believe V1 needs to come out. Fixing the pressure anywhere downstream of the HX1 leg using the new valve will determine the leg's flow rate, as the pressure drop across HX1 is then defined. The upstream pressure at node "Branch1" is known Pus= pump discharge pressure - losses to "Branch1". Pressure downstream of HX1 set by valve Pds. Flow in HX1 = (Pus-Pds) x K[sub]HX1[/sub].

Mark. I'm going to need you to buy me a beer.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[katmar]

Fixing the pressure anywhere downstream of the HX1 leg using the new valve will determine the leg's flow rate

This is true, but the resulting flow rate is unlikely to match the heat transfer requirements. It also offers no flexibility for changing process conditions.

There are two objectives for the control strategy around HX1. The first is to eliminate the vacuum and cavitation problems, and the second objective is to get the correct flow through HX1 to provide the correct degree of heat transfer. In order to achieve both at the same time two valves are required and it would be best to leave V1 where it is and install a new automatic control valve at the bottom of the downleg.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[1503-44]

V1 will control the pressure downstream of HX1 and I have already shown that all flow rates through HX1 are possible while V1 is doing that. There are only two methods of controlling heat exchanged at HX1. 1, Adjusting the flow going into HX1 while maintaining constant water temperature, which is what we are already doing, or 2, Adjusting the temperature of the water going into HX1, while maintaining constant flow. There are no additional means of "providing the correct degree of heat transfer" at HX1, Adding 10 more valves will not do anything more than what can already be done.

Placing an additional valve directly in front of HX1 will only enable flow to HX1 to be reduced, shifting that reduction of flow to the other heaters. That additional pressure drop you add in the HX1 leg from that valve directly in front, will only tend to reduce pressure going to HX1 even more, thereby causing it to start cavitation, upon which you will have to open that valve to recover.
Therefore adding another valve there will be detrimental to HX1 operations. If you must add another valve for some reason I do not know, the best place for it (or them) is in front of the other heaters. Closing valves there will tend to increase flow and pressure to HX1, which was our intent.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[katmar]

@1503-44 We will just have to agree to disagree on this one. If it was my signature in the corner of the drawing there would be 2 valves.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[1503-44]

That would be fine, if you were the one buying the valves and putting it on the other leg.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.