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Pilot Operated Relief Valves

It is a common question asked amongst process engineers on why use a pilot valve for a particular application ? The following article I hope will answer this question and provide some insights into the different types of pilot valves available on the market today and their many features and benefits.

Why use a pilot valve ?

A pilot valve allows higher system operating pressure for maximum process output thus increasing the throughput of your process meaning greater product yield.
Pilot valves are bubble tight when not operating so there are no emissions while the valve is closed.
Set pressure and lift are unaffected by back pressure.
Good for high shock or vibration environments.
Fewer multi-valves required for comparable pressure settings due to body/bonnet bolting limitations of spring loaded relief valves.
Blowdown can be externally adjusted.

Diagram above illustrates seat tightness of a traditional spring loaded relief valve versus a pilot valve illustrating how much closer to set point a pilot valve can operate.

Pilot valves operate on a very simple set up. A sensing line, usually connected into the inlet piping, is connected into the pilot control. Another line is piped from the pilot control into the top of the main valve assembly. The pilot, which contains a small spring assembly, is set at a certain set pressure. During normal operation the inlet pressure passes through the sensing line through the pilot and continues into the dome of the main valve. The inlet pressure in effect is placing pressure onto the piston inside the main valve to keep the valve closed. At set point the spring inside the pilot causes a small piston to slide down allowing the inlet pressure sitting on top of the main valve piston to relieve through the pilot’s exhaust. This causes the main inlet pressure to be relieved through the main valve. The diagrams below illustrate this principle.

There are two main types of pilot control operation:

There are two main types of pilot control operation:
• Pop action.
• Modulating.

Have a look at the following diagram which illustrates how pop action pilots open the main valve fully at set point whilst modulating pilots open the main valve according to relief demand.

Modulating pilots have additional advantages:
They only open the main valve enough to keep the system at set pressure which leads to less wasted product being relieved through the valve.
Less noise generated by the valve when it is required to relieve.

Apart from these operational advantages there are many physical advantages of using a pilot valve:
• More compact compared to conventional spring loaded relief valves.
• Weigh less than conventional spring loaded relief valves for comparable valve sizes.

These two points are clearly summarized in the following diagrams.

Powerflo Solutions has been providing pilot valve sales and technical support to our Australian customer base for over 30 years. We are the sole distributors of the Dresser Consolidated brand of safety relief valves in Australia including the Consolidated 3900 series and 2900 series of pilot valves. These valves offer all of the usual pilot valve benefits already mentioned plus additional options that can be tailored to meet each individual’s needs. Options include:

• Backflow preventers.
• Remote sensing.
• Dual pilots.
• Dirty service options.

Consolidated is the only relief valve manufacturer that offers the ability to retrofit an existing spring loaded relief valve into a pilot valve. Any 1900 series Consolidated spring loaded relief valve can be converted into a 2900 series pilot valve that will give you all of the benefits a pilot valve can offer.

Powerflo - Consolidated have the experience and safety and relief valve product range to meet your requirements. Just ask Shell who are enjoying the benefits of using Consolidated pilot valves for their recent clean fuels upgrade.

For more information on Consolidated pilot valves or any other safety and relief valve requirements you have feel free to contact one of our sales offices located around Australia.

Barry Herrett.
Product Manager – Safety & Relief Valves & Power Products.

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