01 June 2004
Parallel pumps push the process
Trial and error is the original intelligence and scientific method.
By Alagu Sundaram
In process industries such as petrochemicals and semiconductors, there is often a need to operate several pumps of the same capacity in parallel to cater to changing load requirements.
The desired performance attributes for such a pump system include, but are not limited to:
- low energy consumption at high loads
- low energy consumption at low loads
- ease of maintainability
- quick response to disturbances such as pressure fluctuation and power glitches
Here is how the logic plays out in the various control systems—those with programmable logic controllers—that are currently available on the market. Also, see here a novel method to start and stop additional pumps to improve energy optimization of pumps that operate in parallel.
Consider the requirement in the semiconductor industry that demands a maximum of 800 cubic meters per hour (m3/hr) (211,000 gallons per hour) flow of process cooling water at 5 bar for a wafer fabrication. Here are two designs, system A and system B.
System A has five fixed speed (FS) pumps operating in parallel with a bypass valve for control flow regulation.
System B has three variable speed pumps and two fixed speed pumps operating in parallel with no bypass valve.
System A is a relatively simple system with a simple control logic requirement. The control-valve opening controls flow and responds to the pressure difference between the supply and the return headers. Typically the pressure difference stays at about 2 bar. The four pumps can each deliver flow at 200 m3/hr and are sufficient to meet the load requirements.
The fifth pump is a standby to improve maintainability and the availability of the system.
But in system B, pumps one, two, and three operate using variable speed drive (VSD), while pumps four and five are fixed speed pumps. In this case, the control system senses the pressure at the supply header and commands the VSD pumps to either speed up or speed down depending on the pressure feedback from the system.
Though the initial costs for such a system are high, this kind of system can save 30% of the energy costs, because the VSD pumps run at lower than full load conditions. The fixed speed pump is the best candidate to act as a standby pump due to its ability to provide quick response to disturbances and pressure drop situations.
The classical control system currently practiced in several process industries has these attributes:
- Low energy consumption at high loads. Because the system adopts a variable speed drive to control the speed of the pumps to vary the flow, the system is most efficient when it operates at a load of 700–800 m3/hr.
- Low energy consumption at low loads. However, when the flow drops to less than 600 m3/hr, the system still runs three VSD pumps and one FS pump. This is not necessarily the best running combination of the pumps. The VSD frequency of the pumps in this case usually drops to less than 75%.
- Maintainability—monthly maintenance can happen by running the FS pump and stopping the pump that needs work. Technicians can rotate the pumps to perform the upkeep on all the pumps.
- Jerk-free and smooth transition of pumps during the rotation operation. The control system does this by capitalizing on the advantage of the VSDs, which maintain pressure between 4.8 bar and 5.2 bar. It is usually a challenge for the control engineers to decide the proportional (P), integral (I), and derivative (D) parameters to achieve the performance.
- Good disturbance response characteristics of the system, due to the availability of the FS pump, which triggers when the system drops to 4 bar.
- If the load requirements of the system increase to 900 m3/hr, the system requires a sixth pump. The control logic usually changes to five pumps and one standby pump. The choice for the new pump is usually a VSD pump. This is system C.
In addition to these attributes, the control system readily accommodates operator intervention in cases of abnormal system deviations.
So we find that except for energy consumption at low loads and for expandability, the system works fine in all aspects.
In system C where the sixth pump added on to handle increasing loads, the additional pump may not always need to run as the system requirements fluctuate. The load demand may also fall down to 800 m3/hr or even less for long periods of time, and during these times the VSD speeds may drop to less than 80%.
In these situations, the pump performance curves show that operating four pumps instead of five consumes far less energy. At these times the operation engineers face a quandary with two possible solutions.
- Leave all pumps running so the system can cater to higher loads when it is required to do so—but forget about the energy consumption at lower loads.
- Turn off one of the pumps and lose on the reliability of the system—but save on energy costs.
To resolve such a dilemma, do this.
The VSD speed's percentage serves as a good indicator of the system load. The control system—in addition to that described as classical control—must also incorporate other logic.
The percentage of VSD speed to trigger a pump start or stop is—in the end—probably best decided by trial and error, and it depends on several factors:
- number of VSD pumps
- number of FS pumps
- number of standby pumps
When the VSD operates at 75% for prolonged periods and there are three VSDs running, that indicates that the load requirements are relatively low. Also, in these situations, two VSD pumps can do the same job and consume less energy. However, for systems with a higher number of VSD pumps and FS pumps, the 75% criterion is not valid.
But the speed to trigger the starting and stopping of the pumps usually stems from and is based on field experience. Similarly if the VSD speed operates at an average speed of 95% for long time, the system calls for an additional pump to kick in to be ready for any emergency increase in load.
Such a system with online monitoring of the number of pumps is a best alternative for achieving low energy bills and at the same time catering to the varying load demands. This system is particularly useful in industries such as the semiconductor industries where the load requirements vary depending on market conditions.
Behind the byline
Alagu Sundaram has a bachelor's degree in electrical and electronics studies and masters in engineering management. He has ten years of experience in facilities management for the fertilizer and semiconductor industries. Write him at email@example.com.