July, 2008 Subject: Gas Stop Speed/Ratio Valve Calibration Applies to: All GE gas turbines Case study: MS7001B, Speedtronic™ Mark I Related Turbine Tips: Use of Speedtronic Calibrator, August, 2002 GE gas turbines operating on natural gas utilize a combined gas valve assembly. Two valves, usually within the same valve body, control two variables: pressure and flow. Gas pressure “upstream of” the gas control valve (abbreviated GCV) is controlled by the Gas Stop Speed/Ratio Valve (abbreviated SRV). The SRV receives two signals for the purpose of control: turbine speed (NHP) and the inter-valve pressure, P2.
Fig. 1: Gas Fuel Pressure Gages for SRV and GCV
P2 pressure is controlled as a function of turbine speed, NHP. The GE control specifications give a formula for checking the calibration of the SRV which is typically:
P2, psig = R (NHP, in rpm) – Preset Constant P2 = .080 (NHP) – 25.0
(typical formula for MS7001B)
Fig. 2: Gas Stop Speed/Ratio Valve (SRV) – Valve Stem is silver colored
There are three points of interest in the operation of the SRV. 1. Closed-end Over-travel (COT) of the valve. Basically, the fully-closed position 2. Speed (NHP signal) at which the valve begins to open during a fired start 3. Inter-valve gas pressure (P2) at Full Speed, No Load (FSNL) and for on-line operation
Fig. 3: Simplified Schematic of SRV and GCV Operation
Using the formula P2 = .080 (NHP) - 25 1.
P2 = – 25 psig when NHP = 0, the gas pressure. Since this value appears as negative, it means the SRV is fully closed (at the COT)
2. P2 = 0 psig (begins going in positive direction) when NHP = 313 rpm 3. P2 = 263 psig when NHP = 3600 rpm. This is full speed, no load (FSNL) and/or synchronized to the power grid and for all loads. Three points define a straight line. You can plot NHP on the vertical axis and P2 on the horizontal axis of a graph to better visually analyze these three data points.
Fig. 4: SRV operation: NHP is the speed signal and P2 comes from pressure transducer,
Note: A useful tool can be used to test and calibrate the gas control valve circuits. Refer to Turbine Tip for August 2002 regarding use of the Speedtronic Calibrator. The calibrator has several signal sources and ribbon connectors which can be “plugged” into the control panel. Useful signals simulator can be generated including: 0 to 3600 cps for turbine speed (NHP) and 0 to 5.0 volts for the pressure signal (P2) from 0 to 300 psig.
Fig. 5 Speedtronic Calibrator on a Mark I panel in use
Calibrating the gas control valve in an “open loop mode” is very time consuming and requires great patience. You must disconnect the pressure transducer (usually called 96FG) and create an equivalent signal with the Calibrator. Also, you must use one of the Calibrator frequency signals to create the speed reference, NHP.
Fig. 6: Frequency Source for NHP and Current Source for P2 signals
Calibration of the SRV is accomplished with the two circuit boards shown in Fig. 7 (righthand-side cards). SVSB, shown on the right side of Fig. 7, has both gain and offset resistors to calibrate the relationship between NHP and the P2. These two signals can be created using the Speedtronic™ calibrator shown in Fig. 6 above. Note: The actual signals from the gas turbine that create NHP and P2 in normal operation must be temporarily disconnected. Notice now, while in the calibration mode, the “open” loop operation of the SRV is very sensitive. It is suggested that the speed signal for the three key test points noted herein be set and held constant one at a time. Then adjust the P2 voltage signal, increasing it very slowly until the valve holds a steady position. The LVDT feedback signal should cease to move. The dial indicator in Fig. 8 should stop turning when the valve holds steady.
Fig. 7: Circuit Boards for Gas Fuel System Calibration
Sometimes a dial indicator can be used if an additional digital voltmeter is not available to observe the valve stroke with the LVDT voltage, as shown in Fig. 8.
Fig. 8: Dial Indicator checking valve stroke on SRV with LVDT in background
Fig. 9: Pressure Transducer for the inter-valve gas fuel pressure (P2 signal)
For more information, contact PAL Engineering and Dave Lucier at 518-371-1971 at the Clifton Park, NY office or call his cellular phone at 518-330-4801.
