Power Plant Control Station With Start-Up/Trimming Regulator - GE Becker VRP-SB-PID Series Instruction Manual

Power Plant Control Station with
Start-up/Trimming Regulator
Refer to Example Applications #1 and #2 on next pages.
Primary Regulator (Pressure Control)
The Primary Regulator regulates incoming pressure to a
pre-determined pressure required by the power plant. The
Primary Pressure Control Regulator in Layout #1 utilizes a
Globe Pattern Control Valve. Globe pattern control valves are
used for power plants due to their flexible trim styles that allow
for optimization of the valve to the application. Globe pattern
valves use "characterizable" cage trim to achieve different flow
characteristics. Additionally, they are available with multiple
stages of noise trim to prevent the development of excessive
noise when relative pressure drops reach high levels.
An alternative to the globe valve is the T-Ball type control
valve. Refer to the footnotes in following pages for specific
application guidelines. Whether globe or T-Ball style control,
primary pressure control is not complete without one
more element.
Layouts #1 and #2 show primary pressure control implemented
with a Becker VRP-SB-PID Pneumatic Pressure Controller.
VRP-SB-PID's are specifically suited to provide high performance
pressure control when utilized in natural gas fired power plants.
The VRP-SB-PID features zero steady state bleed and eliminates
the need for a pneumatic positioner in most cases. Finally, the
VRP-SB-PID is capable of functioning in a control valve system
that requires pressure control with "flow control override."
Trim-run Regulator (Low Flow Regulator)
The Trim-run regulator is critical for both startup and normal
operation of the power plant. The Trim-run regulator provides
control of extremely low flow volumes that occur during startup
of the power plant. As flow volume increases, the primary will
begin to open and work in tandem with the Trim-run regulator.
During normal operation, both the primary regulator and the
Trim-run regulator will work together to achieve optimum
control accuracy. The Trim-run regulator will accommodate
high frequency/low amplitude fluctuations in the flow. On the
other hand, the primary regulator will handle low frequency/high
amplitude flow changes. The Trim-run regulator of choice for
power plants is the 900TE RedQ as shown in Layout #1, or 2"
Globe valve with VRP-PD as shown in Layout #2. The Trim-run
regulator is coupled with a Becker Model FEP-CH Flexible Element
Pilot for pressure control. Trim-run regulators are typically
designed to utilize a primary and monitor in series.
Trimming Regulator Adjustment
The Trimming regulator is normally adjusted such that the
regulator is open at 20-50%. This allows to keep the setpoint
difference between main control valve and trimming regulator
at the minimum. At the same time, 20-50% keeps the regulator
far enough from the seats to eliminate premature wear.
Both style regulators (Flexible Element with FEP and Globe
with VRP-PD) need to have proportional response only in order
to achieve a droop in sensing pressure. This is accomplished
by an inherent characteristic of the flexible element, and by a
proportional only controller in the case of Globe valves.
© 2016 General Electric Company. All rights reserved.
Example How it Works:
1. Trimming regulator is adjusted at 500 psig (3447 kPa) lock-up.
2. Control valve is adjusted at 495 psig (3413 kPa) setpoint.
3. Power plant starts to take gas. During initial stage only
small amount of gas is needed to satisfy igniters. As a
larger volume of gas is required, the sensing pressure
starts to drop and the regulator opens up. After a 5 psig
(34 kPa) drop is reached the main control valve will come
into operation. At this time theTrimming regulator is only
opened up 20-30%. From now on the sensing pressure
cannot drop anymore because the control valve will satisfy
all volume demands. At the same time any pulsations or
small changes from the plant will be handled by the
start-up/trimming regulator.
4. In order to verify the opening of the flexible element
regulator, a jacket gauge is provided. It is assumed, for
simplicity, that jacket (Pj) pressure is linear in its relationship
with differential pressure (P1 - P2). The following numerical
example indicates how to calculate percent of open for
the regulator: (1)
P1 (Upstream) = 800 psig (5516 kPa)
P2 (Downstream) = 500 psig (3447 kPa)
Pj (Jacket Pressure) = 560 psig (3861 kPa)
% open = Pj - P2 = 560 - 500 = 60 = 20%
P1 - P2 = 800 - 500 = 300
Globe style Start–up/Trimmer Run is provided with standard
(1)
visual linear indicator.
Becker VRP-SB-PID Series Natural Gas Controllers Instruction Manual | 14