Why do I see such a large headloss through my FCV, PSV or PRV?

  Applies To 
  Product(s): Bentley WaterGEMS, Bentley WaterCAD
  Version(s): 08.11.XX.XX
  Area:  Modeling
  Original Author:

Jesse Dringoli, Technical Support
Ben Wilson, Bentley Communities member

Problem Description

I have placed a Flow Control Valve (FCV) and I see a large headloss through it. Can I prevent this?

Reason

As long as the rest of the model is set up to reflect reality, the headloss you see through the FCV is real. If it's very high, it means that the valve would need to close almost all the way in order to restrict the flow to the desired value, while maintaining energy balance for the rest of the system.

Let's look at a simple example:


In both cases, we have 1000 feet of 10" pipe (same roughness), with a head difference of 50 ft, specified by the reservoir elevation. In case A, the model solves the pipe flow as about 3100 gpm, which is the flow necessary to have 50 feet of headloss in a 1000', 10" pipe.

In Case B, we've inserted a FCV with a setting of 1000 gpm. We see that it takes 44' of headloss to achieve this reduction from 3100 to 1000 gpm. Another way of viewing it is that 1000 gpm through 1000' of 10" pipe only results in 6' of headloss, so in order for there still to be a 50' head difference (as dictated by the reservoir boundary conditions), there must be an additional 44' from the FCV.

You could also think of it as an Orifice. At a given total head, in order to have a relatively low flow, the orifice size needs to be relatively small.

Now, if you expect that the FCV shouldn't have to close very much, you'll want to check your model setup, especially boundary conditions.

A similar explanation also applies to Pressure Reducing Valves (PRV) and Pressure Sustaining Valves (PSV.) In these cases, the headloss is what was necessary to achieved the desired upstream or downstream pressure while maintaining boundary conditions. In other words, if you see a large headloss through a PSV, then it means that the PSV would need to close nearly all the way in order to "push" the upstream pressure up to the setting (the more headloss through the valve, the less flow in the pipes to achieve the same total headloss. With less headloss in the upstream pipes, the upstream HGL will be higher)

But I'm trying to limit a demand with an FCV and the above doesn't seem to apply

If you have an FCV with only demands downstream with no other path for flow to supply them, you may see a very large headloss through the FCV, sometimes an absurd number such as thousands of feet or meters.

What is likely happening is that the demand downstream doesn't equal the flow from the FCV and the model ends up solving an equation 120 = 96, which is absurd and the results reflect that. You may also notice a warning message in your user notifications that the model doesn't converge or that "ill conditioning" occurred.

The problem in this case is that this is an improper way of using FCVs. Their logic is fairly crude and won't capture what is really happening in this sort of system.

Generally, FCVs should be used when there is a tank or reservoir element on the downstream side (ie. An "open" system downstream of the FCV). The modelling element is designed to control flow to a fixed HGL boundary downstream of the valve. You should avoid using them in a "closed" system, as it will just return nonsense results.   In a closed system, it is the junction demands that control the mass balance and pipe flow rates.  Placing an active FCV will conflict with the flow rates demanded by the downstream junctions(s) and it will not be able to solve the conflict properly between what flows the junction(s) are demanding vs. the FCV setting.

Also keep in mind that a model is artificial; it is a simulation. So, just because there may be a Flow Control Valve in the real system doesn't always mean you should put one in an artificial hydraulic model! You instead select different model elements to best represent the hydraulic flows and pressures, and often this means using simplifications and different model representations of the pipework from what may actually exist in-real-life to achieve the correct simulation of flows and pressures. A common example is that modellers also don't represent the free discharge of taps and fixtures in the property plumbing and on-site plumbing restrictions, which would demand a reservoir to simulate the discharge-to-atmosphere and several minor loss coefficients.  Instead we just use a junction with a demand as a simpler way to simulate the customers hydraulics.

So for example, in a case where there is a fixed 1000 gpm demand downstream of a FCV, the FCV setting is less than the demand (say 700 gpm) and there's no other source to supply the demand, you could do one of two things:

a)  In a closed system as it exists now, just set the customer junction demand to 700 gpm and remove the FCV.  This will regulate the model flow all by itself.  

b)  Change the customer junction to a Reservoir element (ie. Make it an "open" discharge system), and use the FCV upstream of that set to 700 gpm.

However, method a) generally is much simpler to input, is more reliable and faster for the modelling engine to compute and returns identical hydraulic results in virtually all cases.

Method b) you may only use if the system actually can't reliably supply 700 gpm at all model timesteps, and you want to allow the model pipe flow to drop below 700 gpm in these periods when there is insufficient driving pressure to achieve the flow. Keep in mind that you may also need to represent all the minor loss coefficients between the main and the customer's fixtures as well.

If this doesn't help or still doesn't seem to be the case, take a close look at what else is going on in the system that might prevent the desired reduction in flow as well as provide mass and energy balance across the model.

 

Anonymous
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