11 | GE Oil & Gas
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PID-40, PID-80, PID-125 Explanations
Why a PID-40, PID-80, or PID-125? To understand the reason
why there are different PID models for higher power gas
pressures, we must understand the concept of the “floating”
diaphragm.
In the VRP-SB-PID feedback chamber there is a feedback
diaphragm that introduces the feedback force to the controller.
A closer look at the mechanisms that hold the diaphragm in
place would show that it is “floating” - or not truly connected to
the spring chamber or VRP-SB-CH body. The diaphragm works
by transmitting forces through pistons that are connected to the
spring chamber and VRP-SB-CH body. The only thing holding this
diaphragm in place are the compressive forces from the spring
chamber and VRP-SB-CH body. If we attempt to put 125 psig
power gas through into a PID-40 there would not be enough
of a compressive force to hold the feedback diaphragm in
place, and it would separate. For the PID-40, the pressure
that would cause this separation is 54 psig. For the PID-80
and PID-125, the pressures that cause separation are 100 psig,
and 150 psig respectively.
This is an interesting phenomenon because if the diaphragm
separates that doesn’t mean the PID is damaged. All separation
does is cause the controller to lock up, and cease operation. Upon
change to the correct power gas, the controller will return to
functioning completely normally with no damage to the parts.
In order to keep the diaphragm in the feedback chamber
functioning properly, different springs are introduced into the
bottom cap of the PID. These springs provide a different counter
balance compressive forces that keep the feedback diaphragm
operational. However, the stronger the spring inserted into the
bottom cap of the PID, the less sensitive the PID becomes. There
is a small trade-off involved, stronger springs such as the one
in the bottom cap of the VRP-SB- PID-125 can handle the power
gas for the higher pressure applications, but the minimum
setpoint for control is also higher.
The springs in the bottom cap were designed according to
the specifications of typical applications of the PID. One such
example is the standard low pressure spring and diaphragm
actuator application. The typical diaphragm rating for this kind
of actuator is 40 psig, so a VRP-SB-PID-40 is an excellent fit for
this application. The reason being, a VRP-SB-PID-40 has a
maximum power gas of 40 psig, and the spring in the bottom
cap is designed such that it provides the perfect counter balance
force to keep the “floating” diaphragm operating correctly.
For the VRP-SB-PID-80 there are higher pressure applications
such as the Welker Jet Regulator, or high pressure spring and
diaphragm actuators. And for even higher pressure applications
such as ball valves, the pressure needed to control can be
as much as 125 psig. In this case, the model VRP-SB-PID-125
would be an excellent choice. In each of these higher pressure
applications, the springs in the bottom cap of both the VRP-SB-
PID-80 and VRP-SB-PID-125, are designed specifically for the
purpose of keeping the “floating” diaphragm compressed and
fully functional. Refer to page 27 for part numbers and section
views of the 40, 80, and 125 bottom cap configurations.
Adjustment Procedure
1. Dead Band Adjustment
This adjustment is done by converting a VRP-SB-PID controller to
a VRP-SB-CH controller.
A. Adjust supply regulator to desired pressure. The last digits
in the model number represent the maximum supply gas for
that model PID. Supply pressure should be set according to
last digits in the model number, but it can be less than the
maximum. For example, a PID-40 should have the supply
pressure set at 40 psig (276 kPa), and a PID-125 should
have supply pressure set at 125 psig (862 kPa). However,
if the supply pressure to a PID-40 is 30 psig (207 kPa), that
is acceptable, as long as the supply pressure does not go
above 40 psig (276 kPa).
B. Close valve on measured variable line. Adjust measured
variable in sensing chamber to the desired setpoint using
false signal valves on the spring chamber.
C. Turn adjusting screw counterclockwise until it will not turn
anymore. Do not force adjusting screw.
Close Output
Block Valve
P2
P4
P3
P1
Figure 6 - Output Block Valve
D. Close output block valve (figure 6). Open integral (metering)
valve, and derivative orifice to 6 (wide open).
E. Remove tubing which connects integral and derivative orifices.
F. If this is the first time that the unit is being adjusted after
assembly, firstly remove the locking set screw from the
radial hole in the adjustment drum. This may require the
drum to be rotated until the hole containing the set screw
can be accessed.
G. Turn sensitivity drum to the right as far as it will go (in the
direction of increasing numbers). Then turn the drum one
complete rotation to the left. Use the numbers on the drum
as a guide (i.e. if you turn to the right and it stops on “7” then
turn it back to the left until it rotates back to “7”).
H. For a direct acting controller, turn the adjusting screw
clockwise until the output gauge pressure just begins to
decrease, then stop turning. At this point the gauge may
decrease all the way to zero very quickly.
For a reverse acting controller turn the adjusting screw
clockwise until the output gauge pressure just begins to
increase then stop turning. At this point the gauge may
increase to the maximum power gas very quickly.
