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Full speed in washdown? Not so fast!

Variable frequency drives (VFDs) provide a better way to manage energy and water.

By Ron Crutchfield and Gary Jacott, Motion Industries March 21, 2018
Many food processing plants have pumps in place for their cleaning system; pumps which are used for washdown in the plant. Many of the pumps are original, or the system design is original to the plant. These systems often were designed when the plant had less production and was not as concerned with water and energy usage as are the plants of today. Now, plants are finding ways to conserve water and energy by making adjustments to their current systems or implementing new systems.
Some plants have added pumps or switched to larger pumps in the original system and the pumps run at full flow and pressure when in operation, whether the plant is using an individual gun and hose or several guns and hoses during the clean-up or washdown process. This usage issue has become more of a concern within modern plants where water and energy usage has become a critical operational item. Most washdown process water pumps are designed and selected based on their working full load status, but in practice they are not working at the full load state most of the time. Using a variable frequency drive (VFD) to control the flow rate can solve this, while saving energy and extending equipment life. 
Running pumps full speed was once a non-issue; now, because of the aforementioned reasons, operating them with speed controls such as VFDs has become commonplace for energy savings within the plant. Using controls to regulate the volume and pressure used in the washdown system allows plants to save water by only using the water needed while cleaning is being done, and maintaining the pressures needed to each hose as well. 
This requires some sizing and selection criteria to be evaluated when determining if this type of pump project is of value and acceptable for the plant operations: the number of hoses or guns used during the clean-up process, and pressure and flows required per operation. The pump and system must be sized to meet the requirements when the maximum number operations are using the system, while also allowing for reduced operation when not all hoses are in use. 
Hoses and guns
It’s important to determine the maximum number of hoses, nozzles, guns and other devices used during the plant cleaning operations at any one time and the volume required for each of those devices must be determined. 
For example, say the plant uses washdown hoses with guns/wands which use 11 gallons per minute (GPM) at 400 psi each, and the maximum number in operation at any one time could be 50 devices. This would mean that the pump system must support a minimum of 550 GPM of flow at 400 psi and allow for less if needed, such as when only a few of the operators are finishing their cleaning operation or go to break. 
The number of users would or could drop to few or none, which would require considerably less volumes of water to be provided. This is where a VFD-controlled system offers the flexibility to provide the volume and pressures required for full or reduced capacity as needed. 
When fewer operators are involved, the pumps do not need to continue to run at full speed in order to maintain the flow requirements at the given pressure. The water in the system could maintain pressure while a recirculation line would allow flow continuously through the pumps. Once the pressures are static for 90 seconds (or a determined acceptable time), the pump controls would go into sleep mode—shutting down the motors and waiting for another drop in pressure before starting again.
The second step would be to determine and select the pump or pumps as needed to meet the requirements for the system as determined in the previous step. Simple is better, so if one pump can work for the required flows and pressures of the system, go with that. If not, select the pumps as needed to get to the maximum flows and pressures required. Selecting more than one pump generally gives you greater options for the low end of the spectrum of flows when fewer operations are being applied to the system. This is because the flows are additional and the pressures vary according to the flow in the system.
This type of circumstance has become more common among older and existing plants, where the pumps meet the requirements of the cleaning operations but do not compensate for the “downtime” usage when fewer than the maximum number of pumps are in operation. This wastes energy and can put detrimental loads on the pumps. 
A flexible system
Because the plants and operations vary, these systems can be altered as needed. It is a matter of determining what will work for the individual operation, whether it be one multi-stage pump or multiple pumps staged to perform as needed for cleaning. One possible and commonly effective solution is again, a VFD system, which will control the flow demand by varying the speed of the motors. 
Since these pumps are centrifugal (variable torque or low overload), the following fluid mechanics apply: When an ac motor operates at 75% speed, the power consumption is the cube of 75%, or 42.2% energy usage. Factoring in water conservation, the savings could be even higher for a plant using heated water in their system. The inherent soft start and stop functionality of the VFD also offers energy savings by eliminating the inrush currents common with across the line starting. The VFD also allows varied speed, which means less wear and tear on the mechanical components.
By using a pressure transducer in the discharge line of the pumping system and the VFD control to adjust the speed of the more efficient multi-stage 75 HP pumps, the flows and pressures needed by the system can be maintained. Whether the washdown system is operating at full capacity or as the system requirements are reduced, the pump and VFD can adjust the volumes to meet the required pressure and flows. 
While the system maintains the pressures when the cleaning operation is in process by adjusting motor speeds and starting additional pumps as required to keep the flows and pressures constant, it also allows for the predetermined time allotment. Then the VFD will put the pumps into “sleep mode” and shut down to save energy and water. If an operator were to open the wand/nozzle, thus dropping the system pressure, the pump(s) then ramp up to the flow required to reach the set pressure according to the transducer setting. 
As in the example above, the three pumps were each 100 HP. The system can now operate only one pump at a reduced speed if only one or two cleaning operations were using the system, rather than trying to operate at full volumes and pressures without the controls.
Using efficient, multi-stage pumps decreases the HP per pump, but still provides the higher pressures required for washdown sanitation operations within food processing plants. Adjusting the speed with a VFD provides more control of the energy used according to the requirements of the system. Taking these steps should result in higher efficiency with lower water and energy usage, creating a positive impact for your plant’s bottom line.
Ron Crutchfield has 35 years of experience as a pump engineer, and is currently senior project engineer for Motion Industries Process Pumps and Equipment in Omaha, NE. Gary Jacott has spent 30 years as an electrical engineer, including 20 years specializing in the industrial automation niche.