Valves - How do they work ?

[BRIS]

Help – I have got an insurmountable hydraulic control problem.

I have a single branch pipeline 800 mm diameter 3km long. It is supplied by gravity through a flow regulating valve and discharges into an open tank – top entry.

The discharge level is at 148 0 m above sea level (ASL)

The head at the branch varies between 158 and 175 m ASL. The heads loss at design flow is 5.0 m therefore the head loss across the flow regulating valve varies between 5 and 22.0 m.

The flow regulating valve works in a closed loop control with a flow meter to give a pre set flow (folomatic control)

All very simple you might think but not so.

I have a client’s team including two university professors of hydraulics who are reviewing our design.

The problem is that they cannot understand that a flow control valve works by creating a pressure loss. They insist that a flow control valve and a pressure reduction valve are different animals. Hence they are insisting that we provide either an energy dissipater or an additional pressure regulating valve at the end of the pipeline.

I have attempted to explain by Bernoulli’s equation etc the basic principles of fluid mechanics but to no avail.

How do I explain how a valve works? Any ideas ?

PS I am in Libya

Brian

 

[EGT01]

I suspect there may be a misunderstanding of what is needed from a control standpoint so you may want to review your control needs.

Yes, both a flow control valve and a pressure control valve will create a pressure loss but they do so in response to different variables. In fact a flow control valve provides no pressure loss unless there is flow through the valve.

From your description, it sounds like high pressure downstream is a problem so a flow control valve alone may not satisfy the need to limit downstream pressure. Think about this, if the downstream pressure increases to an undesirable point, with a flow control valve you would have a lower differential between upstream and downstream of the valve, this will tend to reduce flow, the controller will attempt to compensate by opening the valve to maintain setpoint. This will continue until the valve is wide open which happens to be the point of its lowest pressure loss.

From another perspective, if you closed a block valve downstream of a flow control valve, the valve will go wide open trying to maintain setpoint and the pressure downstream of the flow valve will equal the upstream pressure. Hope I'm getting the point across.

As additional comment, if you must have two control valves in series, I think I would prefer to have the pressure control first, followed by the flow control. I think this would allow for more stable operation of the flow controller having a consistent upstream pressure. The setpoint for the pressure control would be the maximum desired downstream pressure.

 

[GGOSCO]

They may be questioning your design for a different reason. Do you have a shut-off valve on the discharge end of your pipeline? If you do, based on your situation, if you were to instantly shut-off the valve on the discharge end, you would most likely encounter severe 'water hammer' and potentially destroy your pipe line.
The only way to control this would be to use a control valve on the discharge end to gradually reduce the process flow to a point where the incompressible fluid column behind the control valve has slowed to a manageable level.

You are correct in what you are saying, and if the above doesn't apply to your situation, please dsregard my comments.

http://www.ggosco.com