Pressure Regulators

There will typically be a small pressure drop through any valve or regulator, and some regulators will specify a necessary pressure drop to pass the rated flow. Thus if the inlet pressure is 9.0 barg, the outlet pressure will be somewhat less. Depending on the size of its passages, "somewhat" may be a little or quite a bit.

Thanks tornado. appreciate the prompt reply.

It is a function of the flowrate and the Cv of the regulator. If there is no flow, the outlet pressure should be 9 barg.

One aspect not yet covered is the control algorithm (bear in mind I am a process engineer not a control engineer)

Pin < 9barg

Valve goes wide open and depending on the configuration and flowrate as already mentioned. It is of course impossible for a valve to create pressure!!!!!

Pout < Pin by a margin

At this time the control algorithm realises the pressure is still too low and tries to open the valve beyond 100% open. This can cause a problem because when the inlet pressure goes above 9barg the controller is 'saturated' and takes time to unsaturate and start to control again. I am a bit hazy on the reasoning as its 20+ years since I studied control theory.

In the old days of mechanical controllers you got round this with anti-reset windup, in todays systems saturate/desaturate limits on Integral action. Without this you probably wont damage the valve but maybe the electronics as you try to put 25 / 30 mA down a 4 - 20mA path

It is possible that modern smart controllers include this function as standard but you would need to ask the vendor or a control / instrument expert

Hi Plutonium

Commonly pressure regulators or pressure reducing valves have a main valve held closed by a light force spring. A diaphragm or piston sensor moves in response to differential force. The desired or set force often is an adjustable spring that loads one side of the sensing device and that forces the main valve open. Gas media pressure on the other side of the sensor will balance the force of the set pressure if adequate.

When the forces are in balance the sensor allows the main valve to be closed by the valve spring. If the regulator is set for an outlet pressure equal to the supply pressure the supply will most likely bleed through when flow is static. With flow and the supply pressure equal to or less than the desired outlet pressure the force required to compress the valve spring, move the sensor etc. will result in a pressure drop at the outlet. This pressure drop may commonly be less at low flow and more with increase flow. With inadequate supply a regulator may act as a low force relief valve. At high flow it may become unstable and vacillate. B-Well, Tom

Although they might be called 'pressure regulators', they are in fact valves to prevent pressure in excess of the set point. The set-point being pre-set or adjustable to a pressure of your choice.

The regulator usually takes the form of a diaphragm acting against a spring. Thus approaching set-point the starts to close thus restricting flow so - that the pressure drops a little. Sensing this slight loss, the valve starts to open again to compensate.

With a properly designed valve, it modulates at the set-point - but in order to remain sensitive - the valve needs to keep moving - which with a good valve there is a slight leakage across the seat causing a slight rise of pressure above set-point valve. The diaphragm now opens an exhaust port that dumps the excess pressure.

The excess pressure is also dumped if the valve is adjusted to a lower pressure. Or sometimes if there is a back pressure from somewhere.

Whatever the case, the valve cannot regulate at a pressure above the supply pressure. And in most cases, a valve has a characteristic pressure drop depending on the flow that is deducted from the supply pressure.

So back to where we came in, the valve can only prevent a rise above set-point.

firstly,

what are you protecting the system from in terms of Maximum Operating Pressures as described by the manufacturers technical data sheets. Its the combination of the total flow loading and the type of appliances being safety regulated that will affect the style and p delta loss calculations for the systems. We normally recommend a fairly high margin due to the fact that a regulator needs time and a resonable margin to be effective.

Your Gas Authorities will have a minimum set of requirements applicable to your area and country.

the internal orifice and spring ratings are calculated based on inlet pressure , outlet pressure and the total flow og the medium accross the regulator itself.

If you supply more information regarding the type of gas and the loads and pressures required I can give you an idea of the style best suited to the application.