The power industry, through its many years of experience, has found that piping arrangements and layout can influence the functionality of a piping system. This section will present specific system guidelines and considerations that will enable the piping designer to minimize that influence.
Main Steam and Hot, Cold Reheat
In any power plant, be it a base-loaded electric power generation station or an industrial facility power plant, the main steam system is the backbone of the installation since it ties together the two most important and most costly pieces of equipment, the steam generator and the turbine, and is also usually the first system designed, giving it the preference in space allocation and routing. The recommendations of the following references should be incorporated in the design of the main steam and reheat steam piping systems.
ANSI/ASME TDP-1-1985, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation (Fossil), American Society of Mechanical Engineers, New York.
1. ANSI/ASME TDP-2-1985, Recommended Practices for the Prevention of WaterDamage to Steam Turbines Used for Electric Power Generation (Nuclear), Ameri-can Society of Mechanical Engineers, New York.
Adherence to the following guidelines will ensure that the system performs its intended function:
All piping in this service should be sloped down a minimum of ¹⁄₈ in/ft (10 mm/m), in the direction of flow. Extensive evaluation and design are required for lines that do not slope in the direction of flow to ensure that condensate is collected and drained adequately.
The final design of the main steam and hot reheat lines should be reviewed, with
consideration for thermal growth, to determine the location of any necessary low-point drains and to ensure that the system can be completely drained in both the hot and cold conditions. When these lines are split into more than one branch into the turbine, each branch should be reviewed for low points. Provide a drain connection in each branch as close as possible to the turbine stop valve. All drain lines and large valve drain ports should have an inside diameter of not less than 1 in (25 mm), to prevent plugging. Main steam piping drains should not be piped together with any other drains from the boiler. In addition, this review should ensure that no condensate can collect in any un-drained portion of the system during shutdown.
Provide a drain pot at the low point of each cold reheat line, which should be fabricated from NPS 6 (DN 150) or larger pipe and be no longer than required to install the level-sensing devices. Each pot should be provided with a minimum NPS 2 (DN 50) drain line and a full-sized, full-ported automatic power-operated drain valve. Each drain pot should be provided with a minimum of two level-sensing devices.
Steam lines that are fitted with restricting devices such as orifices or flow nozzlesshould be adequately drained upstream of the device.
Valves in all steam services should be installed with the valve stem in the vertical upright position to prevent the entrapment of fluid in the bonnet. Where this is not practical, the stem may be positioned between the vertical and horizontal positions, but in no case below horizontal.
Main steam safety relief valves should be fitting-bound to the main steam headers.
Sufficient space should be provided around any steam line to allow for insulation,pipe supports and anchors, thermal growth, machine welding, and maintenance repairs and replacements.
Turbine Extraction Steam
Most steam turbines are provided with one or more low- to intermediate-pressure steam extraction points either for boiler feed water heating or for industrial process service and heating. These systems are extremely critical, particularly from the standpoint of water damage, and must be designed in accordance with the following standards and guidelines:
ANSI/ASME TDP-1-1985, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation (Fossil), American Society of Mechanical Engineers, New York (Ref. 1). ANSI/ASME TDP-2-1985, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation (Nuclear), American Society of Mechanical Engineers, New York (Ref. 2).
The routing should be as short and as direct as possible with consideration for thermal growth and piping flexibility.
Extraction steam piping should be sloped down a minimum of ¹⁄₈ in/ft (10 mm/m), in the direction of flow. Extensive evaluation and design are required for lines that do not slope in the direction of flow to ensure that condensate is collected and drained adequately.
Bleeder trip valves must be located as close to the turbine extraction point as possible, while at the same time keeping the total volume of the system within the turbine manufacturer’s recommendations.
When extraction steam piping is routed through the condenser neck, an expansion joint must be provided in each line and located at the turbine nozzle. The bleeder trip valves in these lines must be located just outside the condenser neck.
A drain should be located at the low point in the extraction pipe between the turbine and block valve and routed separately to the condenser. A power-operated drain valve should be installed in this line that opens automatically upon the closure of the block valve in the extraction pipe.
There should be no bypasses around the extraction line shutoff or nonreturn valves.
Unavoidable vertical loops which create low points in the piping downstream of the bleeder trip valves must be provided with continuously drained drip pots.
Provide a minimum of five diameters of straight pipe downstream of all bleeder trip valves.
Provide maintenance access to all bleeder trip valves including any miscellaneous platforms, if needed.
Condensate
The condensate collection system from the condenser hot well presents a unique set of parameters since we are dealing with water at slightly elevated temperatures and at a vacuum pressure. These conditions make the condensate pump suction piping susceptible to flashing and cavitation. The following guidelines apply to the design of condensate pump suction and discharge piping:
Where two or more condensate pumps are used, the individual runs to each pump must be similar, and if a suction manifold or header is used, the individual pump suction lines from that manifold or header must be similar.
When the manifold or header is larger than the pump suction size, the manifold or header should be made up of full-sized tees and eccentric reducers, flat side up.
Each individual pump suction run should be sloped down a minimum of ¹⁄₈ in/ft (10 mm/m) toward the pump and be self-venting back to the condenser.
Provide a minimum of three to four diameters of straight pipe in the pump suction line; in addition, these lines must be fitted with expansion joints and start-up strainers.
The condensate pump discharge check valve must be located below the hot well water level and be continuously flooded.
The discharge header outlet should not be located between the pump discharge connections to the header, to avoid a counter flow condition.
The condensate pump recirculation control valve should be located at the con-denser nozzle.
Feed Water
The boiler feed water pumps normally take suction from the deaerator storage tank, discharge to the feed water heaters, and then supply the boiler. Here, too, the designer has to deal with the possibility of flashing fluid and must ensure that the deaerator storage tank is located at an elevation that will provide sufficient net positive suction head (NPSH) at the pump. The following guidelines apply to the design of this piping:
The pump suction piping from the deaerator storage tank should drop vertically, avoiding any long horizontal runs of pipe. If short horizontal runs are unavoidable, they should be angled vertically down.
A minimum of 3 diameters of straight pipe is required at the pump suction. The pump suction strainer may be located in this run of pipe.
If a reducer is required at the pump suction, it must be eccentric and installed with the flat side up.
The feed pump discharge swing check valves should be located in horizontal runs of pipe only.
The feed pump re-circulation line control valve should preferably be located at the deaerator storage tank. Horizontal runs are to be avoided in this line at the tank. If the control valve is located in a branch from the pump discharge, the line downstream of the valve must be continuously flooded.
Turbine Drains
This system consists of the turbine casing drains from the turbine to the condenser, a drain collection manifold at the condenser, or other drain vessel as indicated on the system P&ID. The designer should comply with the following standards and consider the guidelines listed below for the physical design of these drains:
ANSI/ASME TDP-1-1985, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation (Fossil), American Society of Mechanical Engineers, New York (Ref. 1). ANSI/ASME TDP-2-1985, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation (Nuclear), American Society of Mechanical Engineers, New York (Ref. 2).
Turbine drain lines and valve ports should be sized for the maximum amount of water to be handled under any operating condition, but in no case may they be less than NPS ³⁄₄ (DN 20).
Drain lines should be designed for both hot and cold conditions and should slope continuously downward in the direction of flow. Flexibility loops, when required, should be in the plane of the slope or in vertical downward runs.
Continuous drain orifices, when used, should be located and designed so that they may be cleaned frequently and will not be susceptible to plugging by debris.
Steam traps are not satisfactory as the only means of draining critical lines; however, they may be used in parallel with automatically operated drain valves.
No part of any drain line may be below its terminal point at the condenser, drain collection header, or other drain vessel.
Only drain lines from piping systems of similar pressure may be routed to a common manifold.
All drain and manifold connections to the condenser must be above the maximum hotwell water level.
Drainage from other vessels, such as feedwater heaters, steam jet ejectors, and gland steam condensers, that drain water continuously must not be routed to turbine cycle drain manifolds.
Drain lines should be connected at a 45 deg angle to the manifold axial center-line with the drain line discharge pointing toward the condenser. Drain line connections at the manifold should be arranged in descending order of pressure, with the highest-pressure source farthest from the manifold opening at the condenser.
Drain connections to flash tanks must be above the maximum water level in the tank.
Drains from the upstream and downstream sides of shutoff valves must not be inter connected.
Drain lines in exposed areas should be protected from freezing.
All turbine drain drawings must be reviewed and approved by the turbine supplier.
Heater Drains
The heater drains system consists of the feedwater heater drains from one heater to another at a lower pressure, to a drain tank, or to the dump line to the condenser. The designer should comply with the following standards and consider the guidelines listed below for the physical design of these drains:
ANSI/ASME TDP-1-1985, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation (Fossil), American Society of Mechanical Engineers, New York (Ref. 1). ANSI/ASME TDP-2-1985, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation (Nuclear), American Society of Mechanical Engineers, New York (Ref. 2).
Drain piping from feed water heaters without an internal drains cooler must immediately drop vertically to provide as much static head as possible upstream of the heater level control valve. Thereafter any horizontal runs must be sloped down a minimum of ¹⁄₄ in/ft (20 mm/m) in the direction of flow.
Drain piping from feed water heaters with an internal drain cooler may be routed horizontally without sloping upon leaving the heater.
Heater level control valves should be located as close as possible to the receiving vessel, with consideration for ease of access and maintenance.
The heater drain system arrangements must be coordinated with the system engineer for analysis to ensure that single-phase water flow is maintained upstream of the heater level control valves and to determine where downstream velocities may require tees and target plates in lieu of elbows for minimizing erosion.
Heater drain dump lines should enter the condenser at approximately the horizontal center-line of the tube bundle. This location should be coordinated with the condenser manufacturer, who will provide the necessary baffle plates to prevent impingement on the condenser tubes.
Only long-radius elbows should be used in heater drain piping.
The use of reducers should be avoided, except at the control valves, which are generally smaller than the line size.
Compressed Air
The compressed-air systems provide service air and instrument air throughout the plant. The following guidelines apply to the design and layout of these systems:
Refer to the compressor manufacturer’s instruction manual for the recommended relative lengths of intake and discharge piping versus compressor revolutions per minute (rpm).
The compressed-air system equipment arrangement and piping design should be such that the air receiver is the lowest point in the system and any condensate in the system will drain to the air receiver, particularly during periods of shutdown when large amounts of condensate can form. The point here is to preclude any possibility of condensates draining back to the air compressor, where it could cause extensive damage. The compressor discharge piping should be as short and direct as possible through the after cooler and into the air receiver. The compressed-air system distribution lines and risers should originate from a separate outlet connection on the air receiver and should be sloped back to the air receiver.
Compressed-air line header branches should have vertical risers and be drained at their terminations.
Individual service branches should be taken off the top of the headers.
Floor and Equipment Drains
Floor and equipment drains in process plants are actually a process system and should be considered in the overall equipment layout design process. All equipment should be carefully evaluated for drain requirements, and a drain hub should be provided for pipes that produce continuous flow. The equipment drain system requires a thorough understanding of the equipment and system function because the drains are designed and issued for construction prior to receipt of finalized vendor information. Drain hubs should not be located more than 6 in (150 mm) away from the equipment pads, to prevent them from becoming tripping hazards.
Sump Location and Pump Discharge
Piping and plant layout should always consider the sump locations as significant due to their impact on overall construction. Civil structural problems can occur when the sump size and location are not evaluated prior to determining a final sump location. Sump discharges should have a non-return valve to prevent the draining of the discharge piping. This is required to keep the discharge line solid and to avoid water hammer.
Fire Protection
The fire protection system usually consists of two or more fire pumps taking suction from the fire water source with the discharge of each pump independently connected to the underground fire main and as widely separated as possible. The underground fire main loop shall completely encircle the plant and may serve multiple sites if cross-connected between units. The National Fire Protection Association codes and the following guidelines may be used to design and lay out the yard fire main loop:
Locate the yard fire main such that all fire hydrants will be a minimum of 50 ft (15 m) from any building or structure whenever possible.
The underground fire main shall be sectionalized in accordance with NFPA code using post indicator valves.
Post indicator valves shall be provided on each side of any fire pump discharge connection into the fire main loop.
All fire protection system branches from the yard fire main loop shall be provided with a shutoff valve located not less than 40 ft (12 m) from the building or structure being served.
Two-way fire hydrants with individual curb boxes should be provided at 250- to 300-ft (75- to 90-m) intervals along the yard fire main loop.
Water fire-extinguishing systems within any building may consist of automatic sprinkler systems, spray systems, deluge systems, and hose stations, as determined by the project engineering group. The following guidelines shall apply to the design of these systems:
Large areas, such as below the turbine operating floor, should be divided into sectors each served by an individual branch from the yard fire main loop.
Each sector should be controlled by an exterior post indicator valve and an alarm check valve or automatic valve located inside the building.
The maximum area served by any one alarm check valve or automatic sprinkler valve shall not exceed 25,000 ft2 (7620 m2).
The maximum number of sprinkler heads in any sector shall not exceed 275.
Provide automatic wet sprinkler systems in the area of the tube oil system below the turbine operating floor and in the ceiling of the clean and dirty tube oil storage tank room.
Separate water spray systems should be provided in the area of the tube oil system, in addition to the wet sprinkler system noted above, and in the area of the hydrogen seal system.
Standpipes and hose stations should be provided in accordance with the NFPA code as a complement to the automatic suppression systems noted above.
The hose stations on any given floor should be fed from above to avoid creating a series of unvented high points.
Cooling Water Systems
There are several types of cooling water systems utilized today in the engineering and design of power generation, petrochemical, and industrial plants. The most common system in use for many years in power generation was the direct use of the water from the nearby river, bay, or ocean. In this system a water intake structure is located along the shoreline and includes as a minimum circulating water pump(s), piping, both fixed and traveling intake screens, and the necessary crane facilities for the removal, replacement, and maintenance of the pumps and their motors. The intake screens are provided to prevent fish, crabs, and other debris from entering and damaging the pumps. In addition to this main cooling water system, there may be one or more service water systems for other equipment throughout the plant. The following guidelines apply in the design and routing of these systems:
Where butterfly valves are used, follow the guidelines provided for valves. Any given heat exchanger inlet and outlet valves should be located close together for balancing the system.
Avoid unnecessary vertical loops in any closed cooling water system. This type of system will usually include an expansion tank, which should be located at or above the highest point in the system, and the outlet from this tank should be piped directly to the pump suction.
For piping at centrifugal pumps, follow the guidelines provided for piping of centrifugal pumps.
Consult the Hydraulic Institute standards and the pump manufacturer’s guidelines for layout and arrangement of deep-well type of pumps.
Since the temperature in these systems is not high and does not vary widely, piping offsets to accommodate thermal expansion and/or contraction are not of paramount importance.