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How Does a Direct-Operated Pressure Regulator Work? (Part 1)

Regulators are everywhere. Most people pass hundreds of regulators feeding gas to houses and businesses every day while commuting but don’t pay them any attention. They are an invention that we take for granted. Just 135 years ago, pressure control was a manual, time-consuming process that is now automated by a simple, mechanical device. The following post discusses the purpose of pressure regulators, the components that make up a regulator, and how these components work together.

Purpose of Regulators

Pressure regulators are self-contained valve and actuator devices, the purpose of which is to reduce pressure and maintain a constant outlet pressure despite varying flow demand. They are self-contained because these purely mechanical devices can control pressure without external power sources and without exhausting any process fluid. Direct-operated regulators, also known as self-operated regulators, are the simplest type of pressure regulator and are the most common due to their simplicity and economy.

 A typical gas application is shown below. The house’s demand for gas varies throughout the day as the heater, stove, and hot water heater cycle on and off. As demand increases, the regulator must increase flow to the house by opening. If it didn’t, the pressure between the regulator and house would decrease. The opposite is also true - as demand decreases, the regulator must restrict flow so that the pressure to the house doesn’t rise. Thus, pressure indicates whether the regulator is supplying the exact amount of gas that the house is demanding.

  3 Essential Elements of a Regulator

Prior to 1880, pressure control was achieved by having a person throttle a hand valve while watching a pressure gauge. In 1880,  William Fisher invented a device to automate pressure control called the pressure regulator, an invention which has improved our daily life. He recognized that the function the valve, the pressure gauge, and a human operator were performing could be performed mechanically by combining three components. The first component is a valve similar to the one manually operated by a human. It consists of a valve body, orifice, valve plug, and stem as shown below. As the stem is moved upward, the valve plug moves toward the orifice, restricting flow. Flow through the orifice is increased by moving the stem downward, away from the orifice.

 The second component in the regulator design replaces the function of the pressure gauge and provides feedback to the regulator on whether the house’s flow demand is being matched. This component is typically a fabric-reinforced sheet of rubber called a diaphragm. It is connected to the valve plug by the stem and it will modulate the valve plug position based off the pressure it senses through a piece of tubing, known as a control line, connected to the downstream piping. If the flow demand from the house decreases, the pressure between the house and regulator will increase, causing the diaphragm to inflate upward. The upward movement of the diaphragm moves the valve plug closer to the orifice, restricting flow which is exactly what needs to happen when the house reduces gas usage. If the house’s gas demand increases, the controlled pressure will decrease, causing the diaphragm to deflate downward. The downward motion of the diaphragm and valve plug opens the regulator farther which is exactly what is needed to match the increased demand from the house.

The third and final essential component of a regulator replaces the human operator. Modern regulators use springs to apply force on top of the diaphragm needed to open a regulator and an adjusting screw to allow the user to adjust the pressure the regulator will ideally control, known as setpoint.

A direct-operated pressure regulator is the combination of the valve, diaphragm, and spring components. Now to illustrate how a direct-operated regulator functions, we can use William Fisher’s regulator in the hypothetical residential gas application with 60 psi inlet pressure to the regulator and it is set to control pressure to the house’s heater, stove, and hot water heater at 1 psi.

Droop

The only way the internal parts of a regulator will move in any direction is if the diaphragm senses a change in that outlet pressure. The importance of this regulator performance characteristic is frequently misunderstood. If the regulator senses decreasing outlet pressure (due to an increase in flow demand), the subsequent decrease in upward force on the diaphragm will move the valve plug down, away from the orifice. This results in additional flow in an attempt to match the increased demand. If the regulator hasn’t opened enough to satisfy demand, the downstream pressure will have to decrease further for the regulator to open more. The term used for this characteristic is droop.

 When flowrate versus outlet pressure is plotted for a direct-op, it looks like the below chart. Setpoint is made at low flow (usually 5-10% of maximum), but that is the only flowrate at which outlet pressure will precisely equal setpoint. If the regulator needs to close more, outlet pressure will have to increase. If the regulator needs to open more, outlet pressure will have to decrease. Some mistakenly think that this is a speed of response issue where it is simply a brief change in the controlled pressure but it will recover back to setpoint at steady state; however, this is not the case. Using the below performance curve as an example, outlet pressure will only equal the 1 psi setpoint when there is 50 scfh flow demand. Any time the flow demand is greater than 50 scfh, outlet pressure will droop below 1 psig. Direct-operated regulators are purely mechanical products that require a change in outlet pressure to function.


Regulator manufacturers could simply publish the wide-open flowrate (500 scfh in our example) but at that flowrate the outlet pressure is 0 psig. The only time this value is used is when sizing a relief valve and need to know the maximum flow the regulator can pass if it failed open. Fisher publishes this value in every pressure reducing regulator bulletin as a wide-open Cv flow coefficient.

 Most applications require outlet pressure to be maintained much closer to setpoint so instead flowrates are published at a specified accuracy; ±20% is common for direct-ops. On the graph below for ±20% accuracy, 275 scfh flow would be published because that is the maximum flowrate before the regulator’s performance droops outside the ±20% accuracy bounds. If the flow demand is less than 275 scfh, the outlet pressure will be closer to setpoint. Likewise, if flow demand exceeds the published 275 scfh, it doesn’t mean that the regulator doesn’t have enough capacity; it simply would be less accurate. In this instance where the required flowrate is higher than published but the application can tolerate more inaccuracy, calculate the maximum that the regulator can flow using the advertised wide-open Cv. If the required flow demand is greater than the published flowrate but less than the calculated maximum, then you can be confident that the regulator will flow more than is required.

 

 Click here to continue to Part 2.

3 Replies

  • Dear Nathan, first of all congratulations for your post. I am studying an application with water where I need to reduce the inlet pressure about 10 bar to an outlet pressure between 4,1 and 2,1 bar-g. The problem is that we have a great rangeability as the flow varies from 11 lpm to 180 lpm. I am evaluating the model MR95 from Emerson, however no bulletin that I have consulted informs the minimum controllable flow. The curves in your post suggest that it is possible to control the flow until zero. Is this understanding correct? What about the bathtub stopper effect? How can I evaluate the minimum controllable flow for the pressure regulators as the bulletins does not inform and the FSM does not allow us to evaluate the regulators as we can do with the control valves ? Thank you in advance !
  • In reply to Marcos Araujo:

    Hello Marcos!

    The minimum controllable flow (also known as turndown) depends on application and proper installation.  Because of the variation in customer installation practices, Emerson does not publish values for turndown ratio. We recommend selecting the smallest body size or orifice that will meet the max flow requirements.

    As a rule-of-thumb, many use a 100:1 turndown ratio when sizing and selecting self-operated regulators. 100:1 turndown ratio means that the product will provide stable control down to 1% of the regulator’s flow capability.

  • In reply to Nathan Wilhelm:

    Hello,
    As Emerpol we deal mainly with natural gas installations and my experience with spring operated regulators operating with water is very limited. My notes below must not be treated as an advise for Marcos for this specific case.
    The issue of regulator turndown is really important. It seems that the spring operated regulators (usually spring open type) can tolerate much higher turndown than the pilot operated regulators. Also, it seems that boot style regulators (one path control) can operate better with small openings than the hard-plug regulators (usually: two path control). As far as I know the Emerson recommendation is to keep the lowest flow not below 5%-10% of Cg max of a pilot operated regulator. In cases when the predicted minimum opening is less than ca 5% of Cg max we suggest to our customers some special arrangements of the reducing line: additional "summer line" of smaller diameter, additional smaller bypass regulator etc. Quite often this suggestions are not welcomed by designing engineers whose awareness of the turndown issue is limited. When we deal with sensitive industrial installations (turbines, gas engines, processing plants etc.) the issue may be of a critical importance. What is also important in these cases is the buffer volume between the regulator and the final installation.
    Summing up: we all need advises and technical support in new challenges. I believe that it would be very useful to built a reference database on best regulating practices based on experiences from suppliers/users from all over the world. What do you think?