Direct acting pressure regulator
with safety shut off valve

Regulator function

Natural gas pressure regulators are designed to maintain outlet pressure at a constant level independently of fluctuations in inlet pressure or gas flow. Under zero-flow conditions, regulators close tightly.

Pressure regulators for natural gas can be found in several different design solution. For situation where quick regulator response is used, like on gas trains and on other places where sudden change of pressure is possible, direct acting pressure regulators are required. Also, very common is use of pilot operated pressure regulators, which you can read about here.

Direct acting pressure regulator is one that doesn't include pilot in its regulating circle. These regulators are often having larger regulation error, in terms of RG, than pilot operated pressure regulators. One possible construction is described bellow.

A system of levers transmits the movement of the spring loaded sensor and comparator (diaphragm) to the control element. Pressure is regulated by changing the control variable (control spring). The diameter of the outlet nozzle and of the duct connecting the sensor and comparator is designed to produce a Venturi effect that overcomes the force of the spring and causes outlet pressure to rise slightly as the flow of gas increases. At zero gas pressure, the regulator is open, i.e. the controlling force (spring) presses down on both the sensor and comparator and the lever system, so that the control element opens and gas flows through the orifice. Outlet pressure then builds up and generates a force exceeds the latter, the sensor raises the levers while the valve disk starts to close the annular gap at the orifice, thus choking the gas flow and halting the outlet pressure build up. As the outlet pressure downstream of the orifice goes down, bringing the diaphragm compartment pressure down with it, the control element is opened by the force of the spring which now prevails. This interaction is repeated until a balance has been achieved between the control force and the outlet pressure acting on the comparator, which depends on the gas flow. If the control element is damaged, or if the system of levers is jammed, any pressure buildup within the regulator and downstream of the control element will be stopped as soon as the integrated safety shutoff valve responds and cuts off the flow of gas.

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 regulator is thus closed.

Regulator size selection

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

For overcritical relief, the following formula applies:
If: (p_e + 1) / (p_a + 1) > 2
KG=(2 x q_n) / (p_e + 1)

For under-critical relief, the following formula applies:
If: (p_e + 1) / (p_a + 1) < 2
KG=q_n / [(p_a + 1) x (p_e - p_a)] ^0.5

Technical data

For proper regulator selection, except flow capacity in terms of K_g, K_v or C_v, several more technical informations are required:

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:

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