Hi!
I’m trying to calculate the energy produced by a turbine in the given network. The junction J-1 has a specific demand that changes hourly and the max operating point of the turbine is 96 l/s. During some hours the demand is higher than 96l/s so I’ve placed a Flow Control Valve (FCV-7) upstream the turbine in order to control that. The other FCV’s in the network only open or close totally in order to operate the bypass. The problems is when the FCV-7 needs to close a little bit (for example from 120l/s to 96l/s) and the pressure drops abruptly. After that point all the system became with absurdly high negative pressures (- 7 000 000kpa range). And I can’t figure out to fix it. Thank you in advance, Teresa
I'm assuming that the flow is being driven by a demand placed at J-1. What is most 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. There is probably also a warning message in your user notifications that the model doesn't converge.
The problem is with the use of FCV's. Don't use them for this kind of problem. Their logic is pretty crude and won't capture what is really happening in this sort of system. Instead use TCV's and adjust their setting to reflect the way that they really do work.
For those open/closed valves, I would just use control statements to open or close the pipe on which they are located.
Hi Tom,
I am also facing the same problem. There is a tremendous pressure drop across the FCV. In my case, i need to supply 2000 m3/d to a customer. So i installed a FCV before the customer point and set the initial flow rate as 2000 m3/d. I atached asnap shot of my connection. you can see there is a loss of 22.2 m Of H20 across the FCV.
Now, How can we avoid this situation because, clients saying that, "as built is Flow control valve, so you have to use FCV only in the model and run it. "
How to come out of it.
With Regards,
Sree
Hi Sree,
Well you have two problems:
1. When to use FCVs and when not to
2. Educating your client better about hydraulic modelling!
Generally, FCVs should only 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 (quasi) fixed HGL boundary downstream of the valve. You should never use them as shown above 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. Putting an Active FCV in will just 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.
On the second problem of client education, I see this a lot (heck, even my Consultants do this to me). Firstly, a model is artificial..........it is a simulation. It is not real-life, and there has not yet been any network hydraulic model software invented that will perfectly mimic real-life. You can't and shouldn't set up a hydraulic model to be "exactly" the same as what has been built in the field. And yet I see model clients demand the network model have ALL the network input exactly as it exists in real-life, including all the nasty things that models can't simulate very well like hydraulically adjacent tanks, all check valves/reflux valves, all closed off branches, complex and contradictory control logic and then scratch their head as to why the model behaves badly (as do the Consultants who don't know any better)
So, just because there may be a Flow Control Valve in-real-life on the customer's service branch doesn't mean you 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 (the most 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).
In the above example, in order to control the flow to 2000 m3/day then you would do one of two things:
a) In a closed system as it exists now, just set the customer junction demand to 2,000 m3/day 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 2,000 m3/day
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 2,000 m3/day at all model timesteps, and you want to allow the model pipe flow to drop below 2,000 m3/day in these periods when there is insufficient driving pressure to achieve the flow...............mind you, you would also likely need to represent all the minor loss coefficients between the main and the customer's fixtures as well!
Good discussion. I'll also throw in a wiki solution I had written a few months ago on this subject:
communities.bentley.com/.../7944.aspx
This one doesn't cover the case of FCVs fighting with assumed demands with no other path around them though; I'll expand it to include the information from this thread.
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
First of all, thank you, Tom!
In fact I didn’t have any warning message that the model didn't converge but still I removed the FCV’s and replaced the FCV-7 by a TCV. The problem now is that I don’t know how to order it to just let pass a specific max flow. To define a TCV I can input a Discharge coefficient or an headloss coefficient but I don’t know how to transform that in the conditions I want to assure: max flow of 96l/s. I’ve search a little about discharge coefficient but I still doesn’t fully understand what conditions i should give to the TCV for that do the same work as the FCV.
Thank you once again,
Teresa