by This chapter highlights the types of pumps commonly found in industrial plants, along with maintenance and operation considerations for a pump and operation considerations for a pump piping layout.
The two primary piping connections are the suction and discharge nozzles (i.e. Liquid inlet and outlet).The impeller Within the pump case draws the liquid into the pump and sends it out at a high velocity. The impeller shaft is sealed with a stuffing box where the shaft exists the case to prevent the pump fluid from leaking, Drips from wearing seals are picked up in the stiffing box drain. The pump shaft is connected to the drive shaft by a coupling, which is enclosed within protective housing. Both pump and driver are mounted on a common baseplate. Miscellaneous pump leaks that collect within the baseplate during operation are drained through a connection during operation are drained through a connection at the front of the pump.
Pump size and configuration vary of the following reasons:
- The commodity being pumped
- The viscosity of the liquid
- Capacity
- Pressure
- Temperature
- Available head requirements
- Physical limitations
Initial pump piping layouts are done with preliminary information. The equipment engineer supplies the plant layout designer with a catalog cut of the pump the most closely represents the one to be purchased . In many cases, this data does not change significantly if the engineer has made the correct selection . piping layouts are started early in the study phase , when the certified vendor drawings become available later in the project ,minor adjustments are made as required. Dimensions of nozzle locations or baseplate sizes may change slightly but revisions to physical nozzle locations (i.e. from top to side or side to front) do not usually occur when the data is finalized. Working closely with the equipment and system engineers acquaints the principal parties with the exact design conditions and minimizes rework.
PUMP TERMINOLOGY
This section highlights some of the most common terms that the plant layout designer encounters when creating a pump layout.
Allowable Nozzle Loading : The allowable nozzle loading is the maximum amount of stress that the piping configuration may impose on the pump suction and discharge as set by the vendor, client , or code. The pipe stress engineer is responsible for working within this tolerance by coordinating the piping design early in a project and rechecking all calculations before formal fabrication issues of piping drawings are made.
Net positive Suction head: NPSH is one of the most important terms a plant layout designer needs to understand when developing an equipment layout that includes pumps and vessels. The required net positive suction head is a measure of the pressure drop of the liquid as it moves from the inlet of the pump to the eye of the impeller. It is a characteristic of the pump that is generally determined by testing and is expressed in “feet of water “ by the pump manufacturer.
Available Net Positive Suction Head : The available NPSH is the net pressure available in a given system, based on vessel pressure and static head , minus the liquid vapor pressure and functional losses in the system . The goal is to maintain equipment heights and minimize pump suction piping to ensure that the available NPSH is greater than the required NPSH. Insufficient NPSH can reduce pump capacity and efficiency and lead to cavitation damage.
Cavitation : The raped collapse of vapor bubbles that can produce noise , result in a los of head and capacity and create a severe erosion of the impeller and casing surface in the adjacent inlet areas.
API (American petroleum Institute ) pumps : this term refers to the horizontal , single stage pumps found in the petroleum industry . The standard developed by vendors , Contractors , and users entitled “API 610 pumps for general refinery service is used to specify pumps for purchase . To a plant layout designer , an pump is a large , refinery type pump.
AVA (American Voluntary Standard) pumps : This standard , Issued by the Hydraulic institute outlines several pumps with standard dimensions . They are interchangeable for a given size, regardless of who builds the pump with no effect on foundation, Piping design or type of electric motor used.
SAFETY :
Proper consideration for personnel safety around pumps requires piping and valve arrangements that do not obstruct access for operation, maintenance or egress. Care must be exercised not to create tripping hazards with auxiliary piping.
OPERATION :
Pumps normally require minimal attention during operation. Valves however, must be located for easy access; this is particularly true for paired or spared pumps. Where manual valves cannot be operated from grade, chain operators shall be used. If chain operators are not allowed per client specifications, platform access to valves shall be considered.
MAINTENANCE :
Piping shall be arranged in a manner to allow adequate access to the pump without requiring excessive dismantling of the piping system. The coupling between the pump and its driver must be easily accessed in order to align the pump and driver. Pump seal access must also be considered. Piping must be kept clear from above the pump for horizontally split pump casings to allow maintenance. For vertically split casings, access must be provided in front of the pumps. Clearance for forklifts or mobile cranes should be provided for maintenance. In cases where pumps are located in a building or other areas where overhead access is limited, monorails or rigging beams should be considered for removal of the pump and/or motor.
TYPES OF PUMPS :
Pumps are available in many different types. The most common are centrifugal, reciprocating and rotary. Reciprocating and rotary pumps are positive displacement pumps. Centrifugal pumps will usually be one of three types; horizontal, vertical in-line or vertical can type. They may have electric motor or steam turbine drivers. (See Figure 1 for examples of horizontal, vertical in-line and vertical can type centrifugal pumps) Reciprocating pumps may have a direct steam piston driver. Rotary pumps usually have an electric motor driver but may be steam turbine driven. In many cases, the fluid pumped by rotary pumps is so viscous that block valves are not necessary. In that case, the relief valve may not be necessary. If turbine driven, there will be a gearbox between the pump and turbine.
PUMP LOCATION :
Pump location will affect the piping layout and how the piping can be supported. Pumps in flammable service shall be located outboard of overhead Piperack’s or structures. Those in nonflammable service may be located beneath the Piperack (subject to allowance in client specifications). Pumps shall be located as close as possible to the source of suction in order to minimize pressure drop in the system while satisfying piping flexibility requirements and nozzle load allowable. Line size and temperature should be determining factors in routing the piping.
GENERAL PUMP PIPING :
Pump suction piping shall be arranged such that the flow is as smooth and uniform as practicable at the pump suction nozzle. To accomplish this, the use of tees, crosses, valves, strainers, near run-size branch connections, and short radius elbows shall be avoided near the suction nozzle. Suction piping shall be designed without high points to prevent collection of vapors. Suction piping shall not be pocketed.
When pump flanges are flat faced, mating flanges must also be flat faced and the joint made up using full-faced gaskets.
Multiple pump arrangements that connect to a common discharge header shall have the discharges connected to the header such that the discharges from pumps operating simultaneously do not oppose one another.
The suction line for all systems designed to API recommendations that connect to API pumps with end, top or side suction nozzles, or API in-line pumps, shall have a straight run of five pipe diameters (nozzle size) between the suction flange and the first elbow, tee, valve, reducer or permanent strainer (Figure 4).
The suction line for pumps other than API, shall have a minimum straight run length of three pipe diameters. This straight run length should be maximized, but in any case the pump manufacturers recommendations should be followed (Figure 5).
REDUCER TYPE AND LOCATION :
Eccentric reducers in horizontal pump suction lines shall be flat on top in order to prevent any entrained vapors in the liquid from accumulating in the high point and possibly causing cavitation in the pump. Pumps in boiler feed water service operating at close to the vapor pressure of the liquid are particularly susceptible to this problem.
The reducer shall be concentric for overhead piping into a top suction pump. (See Figure 2) Reducers in pump discharge lines shall be concentric and located as close as possible to the pump discharge nozzle. In cases where a combination of nozzle size, nozzle location, pipe size and insulation thickness create flange to pipe/insulation interference, eccentric reducers
may be used to gain the required clearance.
Care should be exercised when using a top flat reducer next to a pump suction nozzle where the change in diameters exceeds 4in / 100mm, as this could result in a disturbed flow pattern into the impeller and cause vibration and rapid wear.
VALVE LOCATION AND ORIENTATION :
Valve handwheels shall be oriented in a manner resulting in good access to the valve and pump. The suction line valve shall be installed with the stem in the horizontal. (i.e. install valve in the vertical run of pipe). Gate valves installed in the horizontal can accumulate vapor in the bonnet cavity and cause cavitation in the pump when the trapped vapor breaks loose.
HORIZONTAL DISCHARGE OFFSET :
If existing steel is not available for support of the discharge piping, or if the check valve must be installed in a horizontal run, then Alternate 1 in Figure 3 shall be used. When discharge piping is horizontally offset, care must be exercised not to block access to the pump coupling, seals or bearings (See figures 3 & 12) .
TEMPORARY AND PERMANENT SUCTION STRAINER INSTALLATION :
Special attention must be given to the location of temporary suction strainers to allow for removal. Figures 4 & 5 show examples of suction strainer installation.
For permanent T-type and Y-type strainers installed in a horizontal suction line, the preferred position of the clean-out connection is 30 to 40 degrees from the vertical making sure that there is enough clearance for strainer removal at grade.
Consideration shall be given to the handling of large “T” type strainer covers, and a permanent handling device (e.g. a davit) supplied if access by mobile equipment is not possible.
COMMON SPARE :
Occasionally, one pump is installed as a common spare between two other pumps in different services. The pump must be manifolded in such a way that accomplishes this. Figure 6 illustrates an arrangement commonly used.
CENTRIFUGAL PUMP PIPING LAYOUT
HORIZONTAL CENTRIFUGAL PUMPS :
Horizontal centrifugal pumps usually fit into three categories:
(1) End Suction -Top Discharge
(2) Top Suction -Top Discharge
(3) Side Suction -Side Discharge
The most common are end suction or top suction. Suction piping shall be supported at grade below the elbow for end suction pumps. The discharge line (top discharge) shall be supported from overhead steel whenever possible to allow as much free area as possible around the pump for operation/maintenance. Figure 7 illustrates how piping may be designed for these pumps.
Typical End Suction Top Discharge arrangement
Figures 4 & 5 show common piping arrangements for end suction – top discharge centrifugal pumps. The suction line shall have a straight run between the suction nozzle and the first elbow, tee, valve, reducer or permanent strainer as dictated by the type of pump and/or manufacturers recommendation.
Side Suction / Side Discharge Pump Piping
Pumps may be single stage or multi-stage. Multi-stage pumps are usually side suction – side discharge. These pumps require significantly more space, and present special layout considerations. The pump suction line for side suction pumps shall have a minimum straight run of three pipe diameters (for non-API pumps) or five pipe diameters (for API pumps) between the suction flange and the first elbow, tee, valve, reducer and permanent strainer. If a horizontal suction line cannot be avoided, then the straight run length should be five diameters minimum for all pumps (Figure 8).
VERTICAL CENTRIFUGAL PUMP PIPING
Vertical centrifugal pumps may be in-line, can (self contained) or sump pumps.
In-line pumps are mounted in the line and supported by the piping as the name implies. A pedestal is often required for larger in-line pumps or where the load is too high for the nozzles to handle. The designer must consider access for maintenance and operation in the same way as for horizontal pumps.
Vertical can type pumps are installed in a concrete cylinder but the process fluid is completely contained in the pump “can.” They are used when there is a high NPSH requirement or at surface condensers. This allows the surface condenser to be mounted at a lower elevation. The same is true for a vessel connected to a vertical can pump. The primary concern for the designer is to provide adequate overhead clearance to remove the pump for maintenance.
Vertical sump pumps are usually used to pump waste products or water from a collection sump. Here again a primary concern is to provide adequate overhead clearance to remove the pump for maintenance. The clearance requirements, between the sump walls or bottom and the pump’s inlet nozzle as well as the pump’s length must be given careful consideration during the layout
phase of the project. The sump design at the pump intake shall be based on Hydraulic Institute Standards.
RECIPROCATING PUMP PIPING
Reciprocating pumps are used when high head is required. These pumps require a pressure relief valve (PRV) to be installed between the pump and the discharge block valve. The PRV can be external, in the piping, or integral with the pump casing.
Due to the pulsating action of reciprocating pumps, the designer must consider space requirements for pulsation dampeners. These are usually furnished with the pump but take up additional space. Pump access is even more important for reciprocating pumps since they require more maintenance than other pumps. Do not install any bend (i.e. 90 degree elbow) directly adjacent to the pump discharge. The discharge pulsation dampener must be installed as close to the discharge as possible. Pipe supports must be given special consideration due to the pulsations.
ROTARY PUMP PIPING
Rotary pumps are used for very heavy or viscous fluids. They deliver a constant pulsation-free flow. Piping for these pumps is very similar to that of centrifugal pumps but is usually characterized by the absence of block valves in the suction and discharge piping. If block valves are used, a pressure relief valve must be installed between the pump discharge and the block valve. The PRV discharge is usually routed back to the pump suction.
PUMPS OPERATING BELOW ATMOSPHERIC PRESSURE
Pumps operating below atmospheric pressure (e.g. Vacuum Tower Bottoms Pumps) present special problems. Since the system operates at a negative pressure and very high temperature, the pumps must be located very close to the suction source. This is often directly below the tower or immediately outside the tower support columns. Pumps located directly beneath the tower can be mounted on a special spring base as shown in Figure 9.
AUXILIARY PIPING
Consideration must be given to lube oil and seal oil systems and any cooling water requirements. Care must be exercised not to block access to the pump seals, bearings, seal pots, starter button stations and motor conduit connection when routing these lines. (Figure 10) The pump data sheet shall always be reviewed to make sure these requirements are not missed. For very large pumps these may be on separate skids.
Firewater deluge piping shall be routed so that it does not interfere with pump operating or maintenance access. If the deluge piping design is sub-contracted, the vendor’s design should be checked to ensure that safety egress, operating and maintenance accessways are maintained.
FIELD WELDS
Consideration should be given to the placement of field fit-up welds in shop fabricated piping 2½” and larger. Early in the project, Plant Design should review the options with Construction and the decision documented.
Options include:
(i) Tack weld the flange adjacent to the pump suction and discharge nozzles to permit piping installation in accordance with the machinery flange fit-up requirements.
(ii) Provide field welds that allow fit-up in three directions.
(iii) No field welds other than those required by spool transportation size limitations.
STEAM TURBINE PIPING
Piping at steam turbines present somewhat different considerations from that of pumps. The piping must be designed to prevent the possibility of introducing a slug of condensate into the turbine, which could destroy the turbine vanes.
The inlet piping must have the block valve or control valve installed in the horizontal run with a drip leg and steam trap upstream of the valves whether the turbine is set-up for manual or automatic operation. In cases where the throttling valve is furnished with the turbine and located on the inlet nozzle, the drip leg and steam trap shall be located immediately upstream. (See figure
11)
Reducers installed in the inlet piping to steam turbines shall be eccentric with the flat side on the bottom to prevent the accumulation of any liquid.
SUPPORT OF PUMP PIPING
It is preferred that pump discharge piping be supported from overhead steel whenever possible. This allows piping at the pump to be removed for maintenance. (See figure 12). The piping layout must permit both suction and discharge pipes to be supported independent of the pump(s), such that very little load is transmitted to the pump casing.
DIFFERENTIAL SETTLEMENT
When differential settlement is a problem, it is preferred that the pump suction piping be supported from the pump foundation. This can be accomplished by extending the foundation as shown in Figure 13.
