The design of a piping system can have an important influence on the operating efficiency and life expectancy of the centrifugal pump. Hence design of suction and discharge piping including operability, maintainability and flexibility requirements should be taken into consideration in layout of pump piping.
NPSH Requirements
Pumps are located close to the equipments or reservoirs from which they take their suction in order to maintain a Net Positive Suction Head NPSH required by the pump. Piping design engineer in early stages of development can influence the selection of pumps based on NPSH limitations. Proper interface with the rotating equipment engineer is important.
Suction Piping
Suction piping sizing and design is the most crucial aspect of pump layout. Many centrifugal pump issues arise on account of poorly designed suction piping. Poorly designed suction piping can lead to air or vapour entrainment, leading to cavitation and vibration in the pump which can cause damage to impeller, pump bearing and seals and ultimately impact the life expectancy of the pump.
Suction piping size should not be less than size of pump suction nozzle. In most cases it is one size larger. Due to suction line being larger than pump suction nozzle, reducers are required in the line. The reducers should be installed as close as possible to the nozzle. In general eccentric reducers are used with flat on the top for horizontal suction lines. This ensures that no air is trapped in the reducer. If the air trapped in the reducer enters the pump during operation, it can cause severe damage to the pumps. Suction piping velocities are normally maintained between 1 to 1.5m/s to limit the frictional losses and maintain good NPSH. Suction piping should be short and as straight as possible with minimum bends and without any pockets. The length of straight pipe from last elbow to the suction nozzle should ensure minimum turbulence at pump suction. The straight run should not include branch connections and valves but may include spool for installation of temporary strainer. The straight length to be maintained varies with the type of pump and is tabulated below. These values are approximate guidelines. Short radius elbows should not be used on pump suction.
Location
Pumps are generally located on foundations at grade level in open air. Very large transfer pumps such as crude oil pipeline transfer pumps may be located in adequately ventilated common shelter. These pump houses are normally equipped with overhead traveling cranes for material handling during maintenance.
In most chemical and petrochemical plants, the pumps are placed below or adjacent to pipe racks. Where possible pumps are arranged with their drives facing the maintenance ways from where the pump drivers can be conveniently accessed for maintenance. For good aesthetics the discharge of pumps are maintained along the same center line. The recommended width for maintenance or working aisle is 2 - 2.5m. This will enable mobile handling equipment to access the pump through the maintenance aisle. Layout should also provide good access to gland seals and couplings which are frequently required to be accessed for maintenance and alignment. This requires piping to be routed such that free access is available on one side of the pump and driver. Some pumps may have their seal oil skids of sufficiently large dimension and may affect the spacing between the pumps.
Pumps handling similar services or with similar level of hazards should be grouped together and provided with adequate separation from pumps that pose a different level of hazard to minimise escalation of damage due to a fire or explosion. High pressure pumps should be separated from process equipment and other low pressure pumps for the same reasons. Pumps handling flammable products should not be located under pipe racks, aircraft coolers and equipment's. For large pumps vibration of pumps may dictate spacing between the pump foundations. The clearance between the pumps will be governed by the maintenance requirements. A minimum clearance of 1m is recommended in most cases.
Other Layout Requirements
- The suction and discharge piping must be adequately supported independent of pump foundation and should be flexible enough to limit the load on pump nozzles within the allowable limits.
- Pump suction should have provision for installation of conical type temporary strainer which is required during commissioning stages.
- Piping should be designed with sufficient space around the pumps with permanent handling facilities like monorail or mobile equipment access to facilitate removal of pump or driver.
- The piping should have removable spool pieces if they obstruct the lifting operations.
- Means for lifting are generally provided where the weight of components to be lifted exceeds 25kg.
- In some cases it may be necessary to use expansion joints in suction or discharge piping or both. A pipe anchor must be provided between the expansion joint and pump nozzle to prevent the pressure loads getting transferred to pump nozzle.
- All valves in pump piping should have adequate operation and maintenance access. Pumps handling flammable or hazardous fluids should not be located in pits to meet NPSH requirements.
Pump Piping Layout Studies
If the suction head available for the pumps is low, end and side suction pumps are the preferred choice. Where the suction source is at a low level, the pump should be located close to the vessel or equipment. In the case below there are two routing options available. Option-1 is the preferred choice as it results in lesser suction piping routing with lesser bends. Where more flexibility is required, consideration should be given to Option-2 routing.
Pump Piping Stress Analysis and Supports
Stress Analysis of pump piping should take into account all possible operating conditions. This includes operating and standby modes of pumps, upset conditions and transients on nozzle loads. The support and flexibility of the suction and discharge piping should ensure that the nozzle loads on the pumps are less than those specified in standards such as API 610 or as per vendor provided values. The acceptable nozzle loads shall be compared against the worst load condition being imposed on the nozzle during the upset conditions. By complying to the vendor or code acceptable values of pump loads the piping engineer ensures that no coupling misalignment is likely to occur due to piping loads.
If Spring Hangers are used in the piping system, the springs should be designed in such a way that the first spring hanger takes the weight of suction or discharge piping, its contents and any insulation if applicable. This philosophy if adopted will relieve the load on the pump nozzle during operating condition while allowing free thermal expansion of the pipe.
To take into account any field variations, it is recommended to provide an adjustable support adjacent to a horizontally oriented pump nozzle.
Dont's: Cold spring should not be used to deal with anticipated thermal expansion in the piping system. Use of expansion joints should be avoided to the extent possible.