Variable frequency drive (VFD) systems help reduce energy consumption and provide smart control of the system with the integration of a pressure transducer to regulate constant pressure. To run a constant pressure system, the VFD must administer an algorithm called a proportional-integral-derivative (PID) loop. Rather than relying on mechanical devices, using live pressure feedback via a transducer—paired with VFD controls—provides protective features on the control side. Three of the protective features that VFDs provide, courtesy of their integrated control systems, are pipe-fill, broken pipe and under-load protection. This article discusses the uses for these protective features and further explains how they operate within VFD systems.
Priming is a necessary step in pump applications that involves large and intricate piping systems (for example, irrigation systems). Problems can arise when trying to prime a system while running PID control. It may run at full capacity or at an indeterminate speed. Operation in this state may cause over-pressurization or system instability. Frequent over-pressurization will loosen pipes and fittings, causing leaks, and will eventually lead to permanent damage to the piping.
|Geyser on Green 17--damage can occur if a leak in a remote area goes undetected and the water is not shut off immediately. Photo courtesy of Bailey Ranch Golf Club, Owasso, Okla.|
To avoid this problem, a preset frequency (pump motor rpm) can be used for efficient priming. The PID function is disabled while the pipe is filling and is activated only when exiting pipe-fill operation. The user setup process is:
- Step 1 – Determine the VFD pipe-fill mode frequency. Start at the minimum motor speed that will allow the pump to provide flow. Increase the frequency until the pump provides an optimal gallon-per-minute rate to fill the pipe system quickly without pressurizing it. (It may take a few tries to find the appropriate frequency).
- Step 2 – Determine the pipe-fill time and exit pressure. Run the system at the frequency determined in Step 1. Note the time required to completely fill the piping system and also note the system pressure when full.
- Step 3 – Program the frequency, full-pipe pressure and delay time into the VFD.
The setting for full-pipe pressure needs to be slightly lower than the actual value. The VFD will switch to normal PID operation when the system pressure is higher than the programmed pressure. The delay time will need to be set slightly longer than the actual pipe-fill time to avoid premature switch over.
Sensing Broken Pipe
When a VFD pump system is running PID (constant pressure) control and the piping becomes damaged, the ideal scenario would be to turn off the pump shortly after the detection of a broken pipe. If the pump were to continue running with damaged piping at full speed and maximum gallon-per-minute volume for a long period of time, the damage that would occur from flooding could be extensive (Figure 1).
VFDs provide broken-pipe protection that will trip (shut off) the VFD when an incident occurs. Typically, broken pipe trips are designed around PID operation. The trip conditions consider the live pressure feedback from a transducer mounted on the pipe, as well as the operating state of the VFD. If all the following conditions are met, the VFD will determine that a pipe is broken:
- The VFD is in normal PID operation.
- The feedback pressure from the transducer is a set (user programmable) percentage lower than the set-point pressure.
- VFD frequency is above a set (user programmable) frequency.
To better understand these conditions, imagine what is detected by the VFD as a pipe is broken: As a pipe breaks, water is pumped out and the pressure in the system immediately drops. The VFD senses this drop of pressure and ramps up the output frequency (attempting to meet the increase in demand and re-attain the target pressure) until maximum frequency is reached. At this point, all three conditions have been met, and the VFD will trip, stopping pump operation and providing flooding protection.
Under-Load (Dry-Well) Protection
A common problem that arises with running constant pressure systems using VFDs is a loss of prime or a dry-well condition. This may occur in the following situations and can damage the pump system:
- In a centrifugal pump system, the water on the suction side is either blocked off, or at a low pressure causing a loss of water prime.
- In a submersible pump system, the underground water well is depleted, which results in a loss of water flow.
Water flow acts as a lubricant and a cooling system for the pump. Without water flow, the ability to mitigate heat is lost, and the system will begin to overheat causing permanent damage to the pump and the piping system. Moreover, the stator of the pump will suffer deterioration due to a lack of lubrication.
To provide protection, the amount of current that the pump motor draws when it is running but not pumping water (the no-load amp draw) has to be known. Programming the no-load amp draw into a VFD allows it to monitor and shut down automatically when the pump current falls below the no-load threshold.