This article provides suggestions on the design and construction of steam inlet piping for condensing and non-condensing turbines, as well as exhaust piping for non-condensing turbines.
Steam Supply Piping Design Considerations
- The steam supply line should be connected at the top of the steam main, and should be provided with a steam trap at the low point near the turbine.
- A Y-type strainer with a valved blowdown to a location where it will not discharge on personnel should be located near the turbine. It should be considered that location for steam blowdown of strainer not endanger personnel.
- A warm-up line, venting to atmosphere with a silencer should be provided in the inlet steam line adjacent to the trip/throttle valve on Special Purpose Turbines.
- For installations in the U.S.A., a porcelain enameled “Caution Sign” per NEMA Z535.1 should be placed at the inlet steam line. The sign should be approximately 7 in. x 10 in. with black letters on a yellow background. For other countries, a sign of similar composition and dimensions, with a similar message in the language of the country should be provided.
- Opening the steam inlet valve when the exhaust valve is closed may overpressure the turbine case. Not all turbine cases are designed to withstand the maximum steam inlet pressure.
- An exhaust head should be provided for turbines exhausting to atmosphere to protect operating personnel from hot water. Similarly, location of the exhaust head should prevent operating personnel from getting splashed by hot water when water load is higher than normal (due to drain plugging or other). A plugged drain line may increase the level of hot water with the potential of splashing personnel working in the surrounding areas.
- Non-condensing turbines that exhaust into an exhaust steam main need not be equipped with an exhaust pressure relief valve, provided the following requirements are fulfilled (all precautions here are to avoid exhaust casing overpressure in case of exhaust valve closure):
- Maximum inlet steam pressure does not exceed 150 psig (1050 kPa).
- Exhaust block valve should be car sealed open (CSO).
- Gate valves used as exhaust steam block valves should be installed with stems in the horizontal position.
- A steam trap should be provided at the low points of the exhaust line of any backpressure turbine.
- Exhaust lines for non-condensing turbines that exhaust to a steam main should be routed individually to the main and should be connected at the top of the main.
- Systems with an exhaust containing a water-sealed pressure relief valve should comply with the following:
- Seal water drain line should be sized adequately for the normal water load handled by the line.
- When a weep hole is provided at the valve outlet line for secondary drainage, this should be located above the water seal level but as close as possible to the valve to ensure proper draining when required (such as in cases when the seal water drain line gets plugged).
- Blowdown valves should be provided for low point drains in the turbine supply and exhaust steam lines, and in the turbine casing. Turbine casing drain should be trapped for turbines requiring automatic unattended startup.
- Two block valves should be installed in the supply line. One should be located at the steam main, the other at the turbine.
- Inlet and exhaust steam valves: Installations should permit the complete drainage of accumulated condensate before valve opening. When installed in horizontal pipe runs, pipelines drains may be installed or the valve may be furnished with a body drain on the inlet side of the gate (or plug). Valves should be positioned with stems extending vertically upward or horizontal, or any intermediate position, providing the stem is above the horizontal centerline.
- A control valve should be provided for turbines with remote starting facility. The control valve should be located near the turbine. A bypass valve should be installed for warming the turbine.
- For general purpose steam turbines with governors not equipped with a manually activated override for testing the overspeed trip system, a bypass with a globe valve should be installed around the steam inlet gate or control valve to permit checking the overspeed trip in a controlled manner. Bypass is not required if turbine inlet connection is 1.5 inch or less.
The steam inlet line downstream of the Y-type strainer should be cleaned of mill scale, weld spatter, weld icicles, and debris that could obstruct or damage the controls or the rotating element of the turbine.