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Determining the inlet and outlet pressure for automatic flow control valve
3

Determining the inlet and outlet pressure for automatic flow control valve

Determining the inlet and outlet pressure for automatic flow control valve

(OP)
Hi All,

For automatic control valve sizing, I need to determine the inlet and the outlet pressure. The setup is as follows:

From the main reservoir, water is pumped through a centrifugal pump to another vessel. In between the centrifugal pump and the vessel there is a valve that can be used to manually regulate the flow rate. My intention is to replace the manual valve with an automatic flow control valve and therefore need to determine the inlet and outlet pressure at the control valve. Could anyone please tell me how to do that?

P.S. Please also see the drawing of the setup attached for more clarity.

Thanks :)

RE: Determining the inlet and outlet pressure for automatic flow control valve

You should have a discharge pressure gauge at the discharge of the pump, use it. Check TP-410 for the calculation. It's fairly straight forward.

Here is a calc I did for something similar.

[ur]https://www.eng-tips.com/viewthread.cfm?qid=430771[/url]

RE: Determining the inlet and outlet pressure for automatic flow control valve

If you want to regulate flow using a control valve, the rule of thumb is that a minimum pressure differential of 5 psi across the control valve is necessary.

Having said that, it is not clear from your post what you are trying to achieve.

The filters should have a minimum and maximum flow rate. This flow range should be within the range of the pump performance curve. Filters are not typically in series either, but operate in parallel.

Why don't you elaborate on what you are trying to accomplish.

RE: Determining the inlet and outlet pressure for automatic flow control valve

You will need the pump curve and a means of calculating the pressure required for your flow rates.

Then the inlet pressure is pump inlet pressure plus pump head at your chosen flow. Outlet pressure is your calculated pressure for frictional losses and static head.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Determining the inlet and outlet pressure for automatic flow control valve

(OP)

Thanks everyone for replying to my question. I don't have much background in designing such systems so my question might seem too basic but I want to learn. I have explained what I am trying to achieve in more details below. Please let me know if there is something else I should mention to clarify my idea :)

@MFJewell
There is no discharge pressure gauge installed at the pump.

@bimr
We have an old filter testing system built by a former employee long time ago. To test the filtration efficiency and capacity of our fiters test dust suspended in water is pumped to load up the filter. By measuring the contamination level upstream and downstream the filter its performance is measured. Ideally the test should be carried out at a constant flow rate but when the filter gets clogged gradually by the test dust the flow goes down as well. At present there is a manual flow regulation valve that needs to be constantly adjusted in order to keep the flow rate constant. I am planning to replace this with some automatic flow regulation system that will automatically open the valve when the flow rate decreases due to clogging of test filter.

@LittleInch
What do you mean by "a means of calculating the pressure required for your flow rates"? can you please elaborate a bit more?

RE: Determining the inlet and outlet pressure for automatic flow control valve

What I meant was that the pressure downstream the control valve needs to be calculated at your chosen flowrate or different rates and in you case a clean filter condition and a dirty condition (usually max 1 bar differential).

There are many online calculators or formulae to calculate this.

You will need pipe sizes, lengths, other fittings to get a good estimate of this figure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Determining the inlet and outlet pressure for automatic flow control valve

What it sounds like you are trying to do, is change the manual balance valve to an automatic flow limiting valve. The way these work, in the HVAC applications I'm used to using them on - is they get sized for either 2 PSI (2-32PSI) or 5 PSI (5-60PSI) pressure drop. Assuming you get the 2-32 option, it will take 2 PSI to get your full flow through the valve. And anytime your system is providing more pressure than is needed (between 2 and up to 32 PSI) than the valve has an internal spring mechanism that automatically adjusts to add the extra resistance needed, so that your flow always remains constant. If your pump provides below 2 PSI, than it won't get full flow.

Basically you size your pump to overcome the minimum pressure required to get full flow through the valve - 2PSI. You have to be specific about what flow you want when you order the valve, as they are tailored to give you the precise flow you want at the given minimum pressure drop. So if you want to increase flow you have to replace some of the valve internals, can't just open it more.

RE: Determining the inlet and outlet pressure for automatic flow control valve

Google "delta-p-valve"

RE: Determining the inlet and outlet pressure for automatic flow control valve

From your description, it looks like the max Cv for the valve will be when all of the following occur simultanously:
a) the the filter has reached the max permissible fouling ( ie max permissible pressure drop across the filter)
b) Flow is the desired flow with pump delivering the published head at this flow and speed
c) Level in the source reservoir is lowest
d) Level in the discharge side vessels is highest.

Work these out and calculate the min upstream pressure at the FCV and the max discharge side pressure. Include frictional losses. It does sound like you need help with this, so if there is no one in your organisation familiar with this, contract this task out a third party consultant. You can use the feedback from this thread to as guidance when checking the consultant's work.

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