Is there a way to alter the pressure drop across a check valve?

Hello Yusuf,

Are you looking at steady state or EPS results in WaterGEMS or WaterCAD, or are you looking at transient results in HAMMER? Are these pressures seen at a specific timestep, or the max pressure? Is this the check valve node element, or from a check valve specified in the pipe properties?

Generally speaking the check valve node element does not include any headlosses except during a transient simulation when the valve is partially closed (see "headloss across check valve node" in this article). If you have the check valve set in your pipe, check the "headloss (minor)" and "headloss (friction)" in the "Results" section of the properties. If the minor losses are high, check your pipe's minor loss coefficients.

If you're observing these pressures at junctions on either side of the check valve, the difference could be due to a difference in elevation, as the pressure is measured as the difference between the hydraulic grade and the physical elevation. It is better to compare hydraulic grade to determine headloss.

You may also want to check the calculated Status field in the Results section of the properties, to see if the check valve might actually be closed (and possibly reversed to what you thought it was - check the to/from field)

If this does not help, please provide a copy of the model for review, with steps to reproduce: Sharing Hydraulic Model Files on the OpenFlows Forum

This is what I'm seeing so far. I haven't done any transient analysis yet. So far only trying to do a steady state in Watergems. 

Thank you. I removed the check valve from the pipe. I'm still confused about how to fix this though. At the check valve node, the hydraulic grade is showing 475 ft and then the start on the pipe immediately after the check valve node is showing 322 ft? Even if the downstream "stop" side is showing 399 ft, why would it matter if my pressure is 475 ft?

If the hydraulics dictate that flow will reverse (try removing the pipe check valve and temporarily morph the check valve node into a junction) then the check valve will close and the downstream HGL will be higher than the upstream HGL because of the closure.

I am not sure if this is what you're saying as I cannot see the orientation of everything, so if you've checked the orientation and confirmed that the hydraulics are balanced (as seen in the Calculation Summary), please provide a copy of the model for review with any steps needed to reproduce: Sharing Hydraulic Model Files on the OpenFlows Forum

It shows balanced?

We still haven't received approval unfortunately to share just yet (still working on it). Is there anything else I can check? I really don't need the check valve for this. I was going to use this model in Hammer eventually and need it for that but didn't know if the model would work correctly in Hammer if it never worked correctly in Watergems in the first place. 

Hi Yusef,

From the screenshots provided so far this appears to be working as expected; the check valve closes to prevent reverse flow, isolating the hydraulic grade on each side. If you need further analysis and cannot provide a copy of the model, please provide these screenshots:

1. HGL and flow at check valve location after removing both check valves and running a test steady state
2. With check valve back in place (just the check valve node element, remove the check valve on the pipe)
   1. Full properties of the check valve node element showing input and "results" section
   2. Full properties of upstream pipe
   3. Full properties of downstream pipe

Hey Jesse,

So I think I've kind of figured out the problem. My nodes on the check valves were reversed. The downstream node was listed as the upstream node. I've changed it and everything has gotten 'better'. The only problem that I'm having now is that the pressure I was seeing before 174 PSI at the check valve, I'm not seeing it now. I'm only seeing 159 PSI. The 174 PSI is very close to what we see in the field so that's what's throwing me off. So far, I've reversed the nodes on the check valve and have decreased the length on the suction line and increased the suction line size (to make it match field conditions) and it has changed some (from 134 PSI to 159 PSI) but not entirely. Do you suspect it's a calibration issue at this point or is there something else that could be in play? I just don't see how the pump would be showing a discharge pressure of 174 PSI and would change to 159 PSI as soon as I reversed the nodes on the check valve.

Hey Jesse,

So I think I've kind of figured out the problem. My nodes on the check valves were reversed. The downstream node was listed as the upstream node. I've changed it and everything has gotten 'better'. The only problem that I'm having now is that the pressure I was seeing before 174 PSI at the check valve, I'm not seeing it now. I'm only seeing 159 PSI. The 174 PSI is very close to what we see in the field so that's what's throwing me off. So far, I've reversed the nodes on the check valve and have decreased the length on the suction line and increased the suction line size (to make it match field conditions) and it has changed some (from 134 PSI to 159 PSI) but not entirely. Do you suspect it's a calibration issue at this point or is there something else that could be in play? I just don't see how the pump would be showing a discharge pressure of 174 PSI and would change to 159 PSI as soon as I reversed the nodes on the check valve.

Yusuf Rashid said:

I just don't see how the pump would be showing a discharge pressure of 174 PSI and would change to 159 PSI as soon as I reversed the nodes on the check valve.

Was the 174 psi seen when the check valve was closed due to the incorrect orientation? If so, that would seem to suggest that your pump was pumping against a closed check valve and would be operating at the shut off head. 

Generally speaking there are a great deal of things that can impact the pressure. It could be a calibration problem (see tips here) where something in the model does not match the field conditions (like demands, element status or pipe roughness), or it could be due to a data entry problem (bad physical parameter entered, wrong orientation, wrong status, problematic controls, etc). 

When looking at pressure, one thing to keep in mind is that the pressure is relative to the node elevation (the pressure of water above the reference elevation). So, it is usually better to compare hydraulic grade, in case there is uncertainty in the node elevation, or which elevation the field measurement was taken from. For example the pressure may have been read at ground elevation but the model could be assuming actual pipe elevations (though in that case you would see model pressures higher than measured pressures). See more here.

I think it was the pump going against the check valve. The number was just so like field conditions that I keep thinking it's something else but I'm pretty sure it was just pumping against the check valve. Now I just need to figure out how to calibrate the model. 

A dumb question, if the calibration is off, would it make a huge difference on the hammer transient results?

Thank you!

Yusuf

Yusuf,

Generally speaking transient simulations tend to be more sensitive to data entry, so an uncalibrated model has more potential to skew transient results than EPS/steady state. However, the degree to which this happens will be different in each case and may depend on the types of transient boundary conditions used. A model that produces acceptable flows and pressures in steady state may not produce accurate transient results if the boundary conditions are significantly different from the real system, due to the way the transient waves interact and reflect.

I would definitely recommend carefully reviewing initial conditions results first and get the model in good running condition that matches the field conditions you're interested in before moving to a transient simulation. There are some good tips in the article previously mentioned.