Abstract
In chemical process industry, cooling water systems are vital services during emergency for plant safety. These systems are not provided with any throttling valve for controls as a foolproof system, and often the CSO valves are provided only with a bypass to control the flow at users. The parallel operation of pumps is necessary to meet the wide operating range in the most economical manner. During a power outage, the optimum number of pumps should operate smoothly to satisfy the emergency flow requirement.
Introduction
Centrifugal pumps are the workhorses in the chemical industry. They are considered reliable equipment. However, selecting the most suitable pump model for all possible operating scenario is important to achieve optimum performance and minimizelong-term operating and maintenance costs.
In vital services, two or more pumps are arranged in parallel to meet the variable flow requirement, but without throttle control. These pumps are also connected to the emergency power supply,since the minimum non-interrupted supply (say 60% of rated flow) is necessary for safe operation of the plant during a power failure scenario.The smooth operation of the pump without vibrations and cavitation must be ensuredduring such emergency or part load operation.
It is important to determine the minimum number of pumps required to ensuretrouble free operation and to meet the optimum emergency power requirement.
The steps followed for selection of pumping equipment are as follows:
Designing the Pumping system (System Head Curve)
The engineer must have a clear understanding of the process and the system in which the pumping equipment will operate. A preliminary design of the system should be developed, including an equipment layout and a piping and instrumentation diagram(P&ID). These preliminary drawings will show the various fluid flow paths for the system operation, preliminary pipe diameters and lengths, relative elevations of system components and all valves and other piping components that will be used to establish the system head losses. These drawings will be used by the engineer to calculate the final piping sizes and pumping system head requirements.
With this information, the engineer can develop system head curves that show the relationship between flow rate and hydraulic losses in the piping system. In determining the hydraulic losses, the engineer must include adequate allowances for future corrosion and scale deposits in the piping system over the plant life. However, it is also important to verify the minimum system head considering the maximum suction head (based on highest level at suction source) without additional margins, to determine the maximum operating point.(Refer Figure 1).
Care must be taken when specifying the required pump characteristics to ensure that all possible system operating flow paths are considered. This combined curve shows the total head required of the pumping equipment to overcome system resistance (Variable head) as well as differential static pressure and elevation (Fixed head).
Bernoulli’s theorem for an incompressible liquid states that, in steady flow without losses, the energy at any point in the conduit is the sum of the velocity head, pressure head, and elevation head.
H = V2/2g + P/γ+ Z
Where;
H = energy (total head) of system, (Nm / N or m)
V = velocity, (m/s)
g = acceleration of gravity, (9.81 m/s2)
P = pressure, (N/m2)
γ = specific weight (force) of liquid, (N/m3)
Z = elevation above (+) or below (-) datum plane, (m)
Calculate (or measure) the fixed system head, which is the net change in total energy from the beginning to the end of the system due to elevation or pressure head differences. An increase in head in the direction of flow is a positive quantity. Next, calculate, for several flow rates, the variable system total head loss through all piping, valves fittings, and equipment in the system.
The flow produced by a centrifugal pump varies with the system head. By superimposing the head-capacity characteristic curve of a centrifugal pump on a system-head curve, as shown in Figure 1, the flow of a pump can be determined. The curves will intersect at the flow rate of the pump, as this is the point at which the pump head is equal to the required system head for the same flow. When a pump is being purchased, it should be specified that the pump head-capacity curve intersect the system-head curve at the desired flow rate. This intersection should preferably be at the pump’s best efficiency capacity or preferred operating range for the model.
Deciding Pumps configurationin Parallel Operation
The number of operating pumps will be determined based on the optimum efficiency (Specific speed), NPSH margin availability and the emergency flow requirement.
One of the first steps in planning for multiple-pump operation is to draw the system-head curve, as shown in Figure 1. The system head consists of the static head Hs and the sum Hf of the pipe-friction head and the head lost in the valves and fittings. The pump head curve is plotted on the same diagram. Combined pump head curves are drawn by adding the flow rates of the various combinations of pumps for as many head values as necessary. The intersection of combined H-Q curve with the system-head curve is an operating point. Figure 1 shows the pump head curve and the combined curve for two pumps operating in parallel. Points 1, 2 represent possible operating conditions for anindividual pump when both pumps are running.
Checking pump selection with only system resistance control
In general, a valve or valves in the discharge line of a centrifugalpump alter the variable frictional head portion of the total system-head curve and consequently the pump flow. The use of a discharge valve to change the system head for the purpose of varying pump flow maintains the pump running at the rated point.
However, in a pumping system without throttle valves, the maximum flow is obtained with a completely open valve, and the only resistance to flow is due to the friction in the piping, fittings, and flowmeter.
The system resistance curve,which intersects the minimum number of parallel pump combination within the acceptable operating range for the pump,shall be verified for following:
- The flow shall be higher than the emergency flow requirement.
- Ensure that the Net Positive Suction Head (NPSH) available at the pump’s suction for the maximum flow is greater than the Net Positive Suction Head Required (NPSHR) to prevent cavitation.
- Driver rating shall be sized for this operating range.
It is important to select a pump that will have its best efficiency within the operating range of the system and preferably at the condition at which the pump will operate most often.
Pump models with steep operating curves and the rated point during normal operation at the left side of the Best efficiency point (BEP) will be required to satisfy this operating range.
From Fig 1, it can be observed that the operating points 1 and 2 are in preferred operating region. The maximum flow point 3 with only one pump running out of two satisfies the requirement of minimum 60% of the rated flow during the power outage. The emergency power requirement is reduced from 372 kW for two pumps operating to around 200 kW for single operating pump satisfying emergency flow requirement.
Few other important aspects shall be considered for designing the system with emergency start of the pump after power outage.
- Transient Analysis
A sudden power failure that leaves a pump and driver running free may cause serious damage to the system. The pump and driver usually have a rather small moment of inertia, and so the pump will slow down rapidly. Unless the pipeline is very short, the inertia of the liquid will maintain a strong forward flow while the decelerating pump acts as a throttle valve. The pressure in the discharge line falls rapidly and, under some circumstances, may go below atmospheric pressure, both at the pump discharge and at any points of high elevation along the pipeline. The minimum pressure head which occurs during this phase of the motion is called the down surge, and it may be low enough to cause vaporization followed by complete separation of the liquid column. Pipelines can collapse under the external atmospheric pressure during separation. When the liquid columns rejoin, following separation, the shock pressures may be sufficient to rupture the pipe or the pump casing. So, it is important to perform surge analysis of the system.To avoid down surge (water hammer) in a piping system, ensure proper pipe sizing. Larger diameter pipes reduce fluid velocity for a given flow rate, minimizing the potential for surge pressure.
When the system is operating with the part load, during system turndown or emergency flow, with lower system resistance and a piping layout that have up and down run, there is probable case that pump cannot provide sufficient flow to prime the siphon. This will cause the vaporization and subject the piping to external pressure. To avoid this, we can provide additional static head by providing seal loop equalized with the termination pressure. This will eliminate negative pressure in the system during part load.
It is also necessary to re-start the pump, to supply the required flow during emergency,within the stipulated time after power outage for the plant safety.If the system contains an appreciable amount of liquid, the inertia of the liquid mass could offer a significant resistance to any sudden change in velocity. Upon starting a pump with an open valve, all the liquid in the system must accelerate from rest to a final condition of steady flow.
If the pump is accelerated very slowly, it would produce an all-friction resistance with zero liquid acceleration varying with flow. If the pump is accelerated very rapidly, it would produce a system resistance approaching a shutoff condition that cannot be realized unless infinite driver torque is available.
A motor having a higher than normal pump torque rating or a motor having high starting torque characteristics will reduce the starting time, but higher heads will be produced during the acceleration period. However, the heads produced cannot exceed shutoff at any speed during acceleration. The total torque input to the pump shaft is equal to the sum of the torques required to overcome system friction, liquid inertia, and pump rotor inertia during acceleration. Torque at the pump shaft will therefore follow the motor speed torque curve less the torque required to overcome the motor’s rotor inertia. It is important to evaluate the relative strength of the power system for accelerating motor to rated speed.
Similarly, the suction system of the pump will also experience acceleration during start-up. The entire volume of liquid in the inlet piping will not accelerate at the same rate over its entire length. Liquid directly preceding the pump will accelerate before the rest of the liquidin the pipe begin to flow. This acceleration may be high enough to cause the pressure in the inlet piping to drop below the vapor pressure of the liquid, causing the pump to cavitate. Long runs of inlet piping will extend the length of time in which cavitation can occur. The suction piping length shall be minimal.The suction piping shall be optimally sized to ascertain enough NPSHA with acceleration loss.
- Reliability in power supply system
The electrical system shall have a reliable emergency power supply. Redundancies shall be provided as necessary to avoid the failure. The power shall be restored within the acceptable time duration. The Fast Bus Transfer (FBT) can be used to switch the emergency power supply.
- Pump Monitoring
Pump health shall be monitored using the vibration probes and performance will be monitored with pressure transducers nearest to the impeller suction and discharge. Thiswill ensure that the pump is operating within the acceptable operating range and not cavitating.
Conclusion
In a pumping system for vital services and emergency operation with centrifugal pumps running in parallel and the operation is based solely on the system resistance, the pump model selection shall consider the minimum optimum number of pumps that can run within its acceptable operating region. The pump shall run smoothly at maximum capacity without excessive vibrations, noise, and cavitation.The pumping system,including piping arrangement, pump driver and electrical supply system,shall also be verified for the transient phase during power failure and restart atpart load.
Maheshwar Gaitonde (BE. Prod. Engg.), has around 33 years’ experience inconceptualization of system, optimization studies, basic and detail engineering. He is a subject matter expert in mechanical at Worley, India, and has worked on various chemical, refinery, petrochemical, fertilizer, oil and gas and special metal projects.
Madhura Vichare (B.E. Chemical Engg.), is process engineer at Worley, India. She has 10 years of experience in basic and detailed engineering. She has worked on various chemical, fertilizer, specialty chemical, oil and gas and polysilicon projects.
