Using VSDs for controlling discharge pressure in Pumping Systems

VSD Constant Water Pressure Setup

The pumping industry has seen a big uptake in the use of Variable Speed Drives (VSDs), which opened many possibilities in pumping system controls, including methods for controlling the discharge pressure in pumping systems.

The Problem

Water pressure system design traditionally includes several controls (such as start/stop points, pressure controllers, sizing to allow minimum runtime etc.) to ensure long lifetimes for pumps and motors. Problems however occur when operating conditions of these systems change, such as system demand, broken lines in the system, pump/motor failure, blockages etc. Without methods for detecting these conditions systems continue to run and could potentially cause physical damage to the pumping equipment and other components within the larger pumping system itself. Even before noticeable damage incurs, substantial increases in electricity usage can also result from these conditions.

The Solution

A properly sized VSD and motor/pump, coupled with a pressure transmitter, provides for a constant pressure system (within the pumping range), maximum energy efficiency (for the given system), and peace of mind knowing the VSD monitors the system, shuts it down to protect it when needed and henceforth protects equipment in the entire pumping system.

A Pressure Transducer typically acts as an information source to the VSD, which in turn use this information to control the energy sent to the motor/pump to slow down or speed up the motor/pump and thereby maintain constant water pressure.  This allows for full control of the output of the system with some of the major benefits as follows:

• Consistency: VSDs are best suited to address variables in demand and still maintain efficiently running systems. VSDs do this by reproducing the AC sine wave at a variable frequency (please see How do VSDs Work for more info), thus controlling the output of the pump without restricting the flow or reducing efficiency of the pump.
• Efficiency: Getting a constant pressure by controlling the electrical energy sent to the motor, rather than restricting the output of the water, VSD’s are able to gain efficiency over other methods, and reduce the utility cost of the system.
• Protection: VSDs offer a wide range of monitoring abilities which is used to protect the system from damage. Below are some of the most common protections used in water pressure applications:
  • Dry Run Protection (Low/No Flow): VSDs establish a normal operating speed that determines how fast the motor needs to run to maintain a “set pressure”. If the required pressure is not maintained within this normal speed range, the VSD assumes there is not enough water and will stop running (protects the pump from dry running).
  • Overload Protection: By setting the maximum amperage based on the motor nameplate, VSDs protect the motor from damages caused by over-current (VSD will shut down and report an error when the condition occurs).
  • Under-Voltage Protection: VSDs prevent damage to motors should there be an under-voltage condition (or brownout) from the power supply.
  • Phase Loss Protection: VSDs protect motors should an unexpected phase loss (or unacceptable phase imbalance) occur from the power supply. VSDs can typically tolerate a large amount imbalance without affecting the output of the VSD to the motor.
  • Other: In addition to these, VSD’s have a multitude of protections built into the drive to protect itself and the pump/motor from being damaged.

Summary

While VSD’s provide a much higher level of monitoring and protection, VSDs cannot make up for poorly selected motor and pump combinations. Solid system design is crucial when installing a pressure-controlled water system. One of the most common issues blamed on VSD systems is asking a constant pressure system to operate on the flat portion of the pump curve. These conditions give VSDs an unwarranted bad name when the real issue is in the design of the pumping system itself. There is no substitute to having a good, well-designed setup by a reputable company with experience with these types of systems.

The large supply of VSD’s (with ever-advancing technology built into them) is however proof that VSDs make sense for improving the reliability and protections available for pumping systems. The use of VSDs in this market is beyond critical mass and its future use in this market will continue to increase.

EMHEATER VSD Constant Pressure Settings

To set up a EMHEATER VSD with a Pressure Transmitter to provide constant water pressure, please see the Constant Water Supply Guide Excel file (available for download) which explains and guides installers through the process of determining the various parameters to be set and the applicable values to be used.

For more information regarding VSDs, please refer to the following related blog posts:

• What is a Variable Speed Drive (VSD)?
• Variable Frequency Drive (VFD) vs Variable Speed Drive (VSD)
• Advantages of Variable Speed Drives
• VSD Industry Applications
• Single-Phase To 3-Phase Converters – VSD – EMHEATER
• Choosing between a VSD and a Soft Starter
• Variable Speed Drive vs Solar Drive (for Water Pumps)
• Variable Speed Drive – EMHEATER – Intro Video
• EMHEATER VSD and Solar Drive Fault Diagnosis – Errors
• VSD – Horticulture – Reduce Costs- ESKOM Fact Sheet
• VSD – Oil/Gas/Petrochemical – Reduce Costs- ESKOM Fact Sheet
• VSD – HVAC – Reduce Costs- ESKOM Fact Sheet
• VSD – Farm Irrigation – Reduce Costs – ESKOM Fact Sheet