10 replies to this topic

[mychemicalromance0425]

Hi, 

 

I am now running some pump sizing calculations. I am using a software, but it looks like generating negative pressure at high point (-2.6 psig) when I checked the hydraulic results. The discharge is 3 ft below the grade and high point is 16ft from grade. It passes through an MMF which has an assigned dP of 1.5 bar. (See system sketch attached)

Is it just ok?
 

Attached Files

•  System Sketch.pdf   18.05KB   20 downloads

Edited by mychemicalromance0425, 17 August 2024 - 05:47 PM.

[fallah]

Hi, 

 

I am now running some pump sizing calculations. I am using a software, but it looks like generating negative pressure at high point (-2.6 psig) when I checked the hydraulic results. The discharge is 3 ft below the grade and high point is 16ft from grade. It passes through an MMF which has an assigned dP of 1.5 bar. (See system sketch attached)

Is it just ok?
 

 

Hi,

 

Please let's know the fluid and the pump's discharge pressure...

[Pilesar]

Vacuum pressure in your pipe at the high point of a loop is possible. If you are discharging to atmosphere as shown on your sketch, you would only see negative pressure at the high point if the pipe to the sink were full of flowing liquid. At low flow rates, the vapors at the discharge end will flow in reverse direction until the pressure at the high point is also at atmospheric pressure. This means you cannot count on the negative pressure at the high point during pump startup either. If the high point in your pipe has a vent valve to help remove noncondensible vapor, the vent valve should be closed after inerts are removed in order to reduce the pump energy required.

[mychemicalromance0425]

Hi, Discharge pressure is 52 psig. Thank you

 

Hi, 

 

I am now running some pump sizing calculations. I am using a software, but it looks like generating negative pressure at high point (-2.6 psig) when I checked the hydraulic results. The discharge is 3 ft below the grade and high point is 16ft from grade. It passes through an MMF which has an assigned dP of 1.5 bar. (See system sketch attached)

Is it just ok?
 

 

Hi,

 

Please let's know the fluid and the pump's discharge pressure...

 

[mychemicalromance0425]

So you're saying at pump startup/low flow, I should install a vent/vacuum breaker valve there? Thanks please advise.

Vacuum pressure in your pipe at the high point of a loop is possible. If you are discharging to atmosphere as shown on your sketch, you would only see negative pressure at the high point if the pipe to the sink were full of flowing liquid. At low flow rates, the vapors at the discharge end will flow in reverse direction until the pressure at the high point is also at atmospheric pressure. This means you cannot count on the negative pressure at the high point during pump startup either. If the high point in your pipe has a vent valve to help remove noncondensible vapor, the vent valve should be closed after inerts are removed in order to reduce the pump energy required.

[Dacs]

If MMF stands for Multimedia Filter, I assume this is upstream of a desalination unit.

 

Static height should be good for 7 psig loss. Excluding piping delP, you lose 48 psi (3.3 bar) in your MMF (52- (2.6) -7). 2x your allowed delP.

 

The numbers you provided are a bit suspect.

 

Do you have any equipment in between? What's the water velocity in your pipes?

 

To address your initial question: Yes, you can have negative pressure in your system, mainly dictated by equipment layout/routing.

 

Your system is not uncommon. My facility has this and we're suffering because of it.

 

I should install a vent/vacuum breaker valve there?

 

 

This will solve the start-up issue, but this will bite you in the ass during normal operation. The vacuum breaker valve will ALWAYS open to break the vacuum at high point and this will cause air ingress in your line, resulting to two phase flow. With the right amount of air ingress, you may have slug/churn flow in that line which can lead to vibration. 

 

If you take out the vacuum breaker and once you start up this pump, the negative pressure at high point can be tolerated. However, if the pressure sits too close to the water saturation pressure (at operating temperature), you may experience vapor lock in this segment and worse, cavitation downstream. Both are undesirable.

 

On the other hand, negative pressure is never a friend of PVC, which I guess the MOC for your pipes for this service.

 

If this is greenfield, try to have your routing to avoid such high point. If it's not possible, introduce a backpressure downstream of your MMF to force the high point to go above atmospheric. If your hands are tied and can't do much, then either (1) tolerate the negative pressure and ensure that segment is designed for vacuum (by adding stiffening rings) or (2) add the vacuum breaker and ensure that segment is designed for two phase flow.

Edited by Dacs, 18 August 2024 - 12:02 AM.

[Pilesar]

The difference in elevation in your example is very small and probably won't matter much. For startup, the pump needs enough head to push the liquid up the hill even if there is a full flow pipe on the downside of the hill resulting in negative pressure at the high point. You should consider all operating cases when designing the pump. You have not stated the fluid. Some fluids may flash in the low pressure and form vapor which can greatly restrict the flow rate. High point vent valves and low point drain valves can be useful for maintenance and do not add much cost when included in the initial piping design. I don't know your system. You probably don't want a constant vent at the high point. Large vertical lines can be self venting. So you may have better system performance if the line is properly sized so that any vapor bubbles are swept along to the exit. You are already modeling the hydraulic system so use the model to see the effect of any changes.

[mychemicalromance0425]

Fluid is just water tho and has very vapor pressure. Actually the material of construction from the MMF to grade is PVC.

. I have a similar pump system as well but it pumps fluid to piperack around 35 ft, then it must go down to below grade for the sumps. It also has this negative pressure at high point. 
Essentially we should not have pressures less than atmospheric at the high point right? But the destination pressure is grade/below grade which is considered to be atmospheric.
 

If MMF stands for Multimedia Filter, I assume this is upstream of a desalination unit.

 

Static height should be good for 7 psig loss. Excluding piping delP, you lose 48 psi (3.3 bar) in your MMF (52- (2.6) -7). 2x your allowed delP.

 

The numbers you provided are a bit suspect.

 

Do you have any equipment in between? What's the water velocity in your pipes?

 

To address your initial question: Yes, you can have negative pressure in your system, mainly dictated by equipment layout/routing.

 

Your system is not uncommon. My facility has this and we're suffering because of it.

 

I should install a vent/vacuum breaker valve there?

 

 

This will solve the start-up issue, but this will bite you in the ass during normal operation. The vacuum breaker valve will ALWAYS open to break the vacuum at high point and this will cause air ingress in your line, resulting to two phase flow. With the right amount of air ingress, you may have slug/churn flow in that line which can lead to vibration. 

 

If you take out the vacuum breaker and once you start up this pump, the negative pressure at high point can be tolerated. However, if the pressure sits too close to the water saturation pressure (at operating temperature), you may experience vapor lock in this segment and worse, cavitation downstream. Both are undesirable.

 

On the other hand, negative pressure is never a friend of PVC, which I guess the MOC for your pipes for this service.

 

If this is greenfield, try to have your routing to avoid such high point. If it's not possible, introduce a backpressure downstream of your MMF to force the high point to go above atmospheric. If your hands are tied and can't do much, then either (1) tolerate the negative pressure and ensure that segment is designed for vacuum (by adding stiffening rings) or (2) add the vacuum breaker and ensure that segment is designed for two phase flow.

[fallah]

Hi, 

 

I am now running some pump sizing calculations. I am using a software, but it looks like generating negative pressure at high point (-2.6 psig) when I checked the hydraulic results. The discharge is 3 ft below the grade and high point is 16ft from grade. It passes through an MMF which has an assigned dP of 1.5 bar. (See system sketch attached)

Is it just ok?
 

 

Hi,

 

With the data provided and lack of the adequate info, seems the pressure at the high point to be lower (around -4 to -7 psi) and the discharge pressure also appears to be lower than 52 psig (the value mentioned in your previous post).

[Pilesar]

Why do you think low pressure at the high point would be a problem? Unless there is something special about your water you haven't told us, I would not be concerned. Low pressure at the high point would be a desirable situation for efficient fluid flow. I would try to maintain the negative pressure by not oversizing the downpiping.

[Dacs]

Why do you think low pressure at the high point would be a problem? Unless there is something special about your water you haven't told us, I would not be concerned. Low pressure at the high point would be a desirable situation for efficient fluid flow. I would try to maintain the negative pressure by not oversizing the downpiping.

 

Because PVC is not normally rated for vacuum conditions, especially for large bore pipes. At the very least, the piping should be rated for vacuum conditions for that segment.

 

The height is also limited by the vapor pressure of water. Go higher and you risk vaporization in that point (aka column separation).

 

Fluid is just water tho and has very vapor pressure. Actually the material of construction from the MMF to grade is PVC.

. I have a similar pump system as well but it pumps fluid to piperack around 35 ft, then it must go down to below grade for the sumps. It also has this negative pressure at high point. 
 

 

 

See my response above.

Essentially we should not have pressures less than atmospheric at the high point right? But the destination pressure is grade/below grade which is considered to be atmospheric.

 

More the reason to believe that this is a seawater system, either for desalination or for cooling.

 

The issue my plant is facing is quite similar with yours. We're experiencing vibration at high point due to air ingress through the breaker valve and we can't just close the valve because the piping (PVC) is not rated for vacuum.

 

If you were to keep the routing, I suggest don't put a breaker valve and just rate the pipe for vacuum condition.