Type 99

H

d

 

 

outlet pipe

c b

e

k

f

inlet

inlet pressure

g

a

outlet pressure

 

 

loading pressure

 

 

atmospheric pressure

 

 

A6814

Figure 2.  Schematic of Type 99 Regulator with Type 61L (Low Pressure) Pilot

Principle of Operation

The key to the operation of a Type 99 regulator is the yoked double-diaphragm pilot (letter keys in this section refer to both Figures 2 and 3 unless otherwise noted). Fast response and accuracy are made possible by the amplifying effect of the pressure-balanced pilot and by the two-path control system. The function of the pilot is to sense change in the controlled pressure and amplify it into a larger change in the loading pressure. Any changes in outlet pressure act quickly on both the actuator diaphragm and the loading pilot, thus providing the precise pressure control that is characteristic of a two-path system.

A typical pilot has an approximate gain of 20, which means the outlet pressure needs to droop only 1/20 as much as a self-operated regulator in order to obtain the same pressure differences across the main diaphragm. Advantages of a pilot operated regulator are high accuracy and high capacity.

Upstream or inlet pressure is utilized as the operating medium, which is reduced through pilot operation to load the main diaphragm chamber. Tubing connects the inlet pressure to the pilot through a filter assembly. Downstream or outlet pressure registers underneath the main diaphragm (E) through the downstream control line.

In operation, assume the outlet pressure is less than the setting of pilot control spring (A). The top side of pilot diaphragm assembly (F) will have a lower pressure than the setting of spring (A). Spring

(A)forces the diaphragm head assembly upward, opening the relay or inlet orifice (C). Additional loading pressure is supplied to the pilot body and to the top side of main diaphragm (E).

This creates a higher pressure on the top side of the main diaphragm (E) than on the bottom side, forcing the diaphragm downward. This motion is transmitted through a lever, which pulls the valve disk open, allowing more gas to flow through the valve.

When the gas demand in the downstream system has been satisfied, the outlet pressure increases. The increased pressure is transmitted through the downstream control line and acts on top of the pilot diaphragm head assembly (F). This pressure exceeds the pilot spring setting and forces the head assembly down, closing orifice (C). The loading pressure acting on the main diaphragm (E) bleeds to the downstream system through a small slot between the pilot bleed valve (D) and the bleed orifice (H).

Normally, excess loading pressure slowly escapes downstream around bleed valve (D) (Figure 3) or

3