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Pilot operated pressure regulator
with safety shut off valve

Regulator Function

Natural gas pressure regulators are designed to maintain outlet pressure at a constant level by equalizing fluctuations in inlet pressure or gas consumption. Under zero-flow conditions, the control element is required to shut tightly.

Pressure regulators for natural gas can be found in several different design solution. For situation where precise pressure regulating is required and flow rates are not changing suddenly, like on gas metering stations, city gates, distribution networks, pilot operated pressure regulators are used. Also, very common is use of direct acting pressure regulators, which you can read about here.

Pilot operated pressure regulator is one that includes pilot regulator in its regulating circle. Pilot regulator is another regulator whith much smaller capacity. Smaller dimensions are enabling fine regulation and accurate setting of main regulator. These regulators are often having small regulation error, in terms of RG, than direct acting pressure regulators. One possible construction is described bellow.

The control element will open whenever the outlet natural gas pressure declines because of increase of consumption, while it will close whenever the outlet gas pressure increases. The energy needed for operating the control element is tapped directly from the flow of natural gas, so auxiliary energy is not required.

The setting of control element of pressure regulator is regulated by the control gas pressure which is tapped between the needle valve and the pressure regulator. This control pressure impinges on the actuator of the governor. Reseting is effected through the closing spring. The control gas pressure is generated in the system, which consists of pilot regulator, a needle valve and the governor.

The outlet natural gas pressure, which is tapped at the measuring point is registered by the by the comparator diaphragm of the pilot regulator. Nominal outlet pressure is adjusted by manipulating the control spring. To increase the pressure, turn the adjustment screw clock-wise. While the outlet pressure is equal to the set pressure gas will pass the pilot regulator, flowing through the needle valve into the outlet pipe. Between the pilot regulator and the needle valve, this movement creates the control pressure which is required to keep the control element in the right position.

If the outlet gas pressure drops below the set value because of an increase in natural gas consumption or for any other reasons, the effective diameter of the pilot regulator orifice increases, more gas flows into the outlet pipe, and the control pressure goes up. This causes the control element to open further, so that the outlet pressure rises again.

If the outlet pressure builds up beyond the set value, the aperture of the pilot regulator is reduced, less gas flows towards the outlet pipe, and the control pressure goes down. The closing spring shuts the control element, and the outlet pressure declines again.

Since the action of the regulator is proportional, pressure will not return to the precise set value from certain positions of the control element. There may be permanent divergences amounting to between 5% at low outlet pressures and 1% at high outlet pressures. These permanent deviations may be influenced with certain limits by manipulating the needle valve setting.

The control pressure can be adjusted either by modifying the intermediate pressure (adjusting the pressure control spring of the inlet regulator) or by re-adjusting the needle valve. Since both manipulations have an influence on the quality and dynamics of the regulation process, they are useful in fine tuning the regulator system to the prevailing conditions.

Whenever the consumption of gas ceases completely, the control elements of the pilot regulator and the governor close tightly, and pressure equalizes on both sides of the control diaphragm of the actuator. In the governor, the closing spring presses the valve seat against the rim of the nozzle, sealing it completely. The pressure developing in the outlet pipe generally is 10% above the nominal in low outlet pressure systems and 2.5% above the nominal in high outlet pressure systems.

Safety shut off valve function

Safety shut off valves will cut off automatically the flow of natural gas through a gas pressure regulator as soon as gas pressure within the protected system reaches a specific upper response threshold.

As the outlet pressure increases, the diaphragm rises against the force of the control spring. The connecting rod engages with the system of levers and releases the actuator and its attached control element on which the force of the closing spring acts. The governor is thus closed.

Regulator size selection

To choose the correct regulator size, the KG characteristics should be computed using the lowest local inlet pressure pe [bar] and the maximum flow rate under standard conditions (p=1.013 bar, t=15 oC), qn [m3/h].

For overcritical relief, the following formula applies:
If: (pe + 1) / (pa + 1) > 2
KG=(2 x qn) / (pe + 1)

For undercritical relief, the following formula applies:
If: (pe + 1) / (pa + 1) < 2
KG=qn / [(pa + 1) x (pe - pa)] 0.5

For application in natural gas system (d=0.6), regulator sizes can simply be extracted from the table.
For all other applications, multiply the KG value computed with a factor f, as follows:

f
Air(d=1)1.29
Nitrogen(d=0.97)1.25
Propane(d=1.57)1.61
Butane(d=2.09)1.87
Town gas(d=0.43)0.85
Natural gas(d=0.6)1

Technical data

For proper regulator selection, except flow capacity in terms of Kg, Kv or Cv, several more technical informations are required:

Inlet pressure[bar]
Outlet pressure[bar]
Operating temperature range[OC]
Ambient temperature range[OC]
Nominal diameterDN
Standard pressure categoriePN
ConnectionFlanged or threaded

Materials

Commonly used materials for pressure regulators working with natural gas and LPG, as non corrosive medium, are carbon steel with nitrile rubber for control diaphragm, like one example given bellow:

Valve bodyC22.8, ASTM A105 Steel
Diaphragm casingS235JRG2
Control diaphragm70NBR
Valve disk seal80NBR
Rods, orifices, etc. X 12 CrNiS 18 8
Safety valve bodyAlSi
Safety valve diaphragm745 N

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