Pump stations are one of the most fascinating points in a water system. A pump converts electric energy into kinetic and potential energy in the form of flow and pressure, enabling us to move water uphill, stabilize pressures, and create a more advanced water system.
However, when energy is introduced into the system without careful coordination with other control elements, the results could be surges, over-pressurization, or pumps operating off their curve – any of which could result in catastrophic damage. And when pumps are fitted with Variable Frequency Drives (VFDs), control valve and pump coordination is even more critical.
Before diving into how valves and pump VFDs work together, it’s important to first understand the many roles control valves can play in a pump station.
The Function of Control Valves in Pump Stations
Depending on the application, control valves may be enough to help pumps operate safely, efficiently, and within system limits. Here are the types of control valves you’re most likely to find in pump stations:
1. Booster Pump Control Valve
Slowly opens and closes to prevent surges on start-up and shutdown. Prevents system shock by gradually introducing and cutting off flow. Also includes a built-in drop check to prevent reverse flow.
2. Calm Network Valves
Slowly opens and closes to prevent surges on start-up and shutdown, but with a dual solenoid modulating pilot system so each portion of the valve stroke can be adjusted. Provides ultimate control of pump flow to create a calm network. Can remain open during operation or be configured for flow or pressure control. Also includes a hydraulic check to prevent reverse flow.
3. Flow Control Valve
Restricts pump flow to a set point. Can use hydraulic controls with an orifice plate or dual solenoids for electronic control. Often used to smooth start-up and shutdown. Usually includes a hydraulic check feature to prevent reverse flow.
4. Back Pressure Valve (sometimes called pressure sustaining or relief valve)
Controls inlet pressure (or back pressure on the pump) to keep the pump at a desired point on its curve. Back pressure control can be used to indirectly control flow. Usually includes a hydraulic check to prevent reverse flow, and optional solenoid control can help with pump start-up and shutdown.
5. Pressure Reducing Valves
Maintains downstream pressure by adjusting flow based on demand. When installed on a pump’s discharge, it restricts pump flow based on downstream pressure. Typically include a solenoid shut-off and check feature in pump control applications.
6. Deep Well Pump Control Valves
Diverts initial flow to waste during start-up to clear the line of air, sand, or odor, before closing to divert flow into the system. Typically paired with a check valve or other control valve, and opens before shutdown to prevent surges in the system.
7. Pressure Relief Valves
Not technically a pump control valve, but often paired with pumps to protect the system and pump from over pressurization. Usually located on a tee flowing out of the system, pressure relief valves open when pressure exceeds normal operating limits.
What is a Variable Frequency Drive?
A pump converts electricity into mechanical energy by turning an impeller, creating flow and pressure in the system. Traditionally, pumps operated at a single speed (either on or off), and relied on control valves to manage surges and regulate flow and pressure.
But here’s the challenge: Even while a control valve can effectively restrict the flow and reduce a pump’s output, a single-speed pump will continue to consume 100% power. That’s wasted energy that results in higher electricity costs and loss of efficiency.
A Variable Frequency Drive solves that problem by allowing a pump to operate at various speeds, matching energy use to demand. By controlling the electrical frequency supplied to the motor, the VFD adjusts pump speed, which saves energy and improves efficiency.
How does a VFD Work?
VFDs manipulate the incoming electrical power by converting AC (alternating current), to DC (direct current), and back to AC. During this process, the VFD changes the pulse width and frequency of the output power, effectively controlling how fast the pump motor spins.
When combined with flow meters, pressure sensors, and control logic, VFDs can:
Ramp pumps up or down their pump curve based on system demand
Maintain steady flow or system pressure
Reduce surges by gradually starting and stopping pumps
Pump Control Valves vs. Pump VFDs
Because both pump control valves and VFDs can control flow and pressure, it’s easy to assume you need one or the other, not both. But that’s not always the case.
Here are a few examples of when a VFD alone may not be enough:
A VFD may help smooth start-up and shutdown surges, but if its minimum frequency still results in surges, a booster pump control valve or calm network valve is still needed.
A VFD can’t divert flow to waste, making deep well pump control valves indispensable.
A VFD can help control pressure but it cannot relieve overpressurization like a relief valve
When a VFD fails, we’ve heard that lead times can be as long as 6+ months. Having the right control valve in place as an emergency redundancy can mean the difference between a functioning or inoperable system in the interim.
This brings us to the heart of this article: coordinating pump control valves and pump VFDs.
How to Coordinate Valves with Pump VFD’s
When valves and a pump VFD aren’t coordinated, they can “fight” for control–and possibly get into a negative feedback loop.
Here’s what it looks like when a pump VFD and Control valve try to control the same pressure.
As illustrated above, a valve controlling pressure downstream of the pump starts to close to reduce pressure. The pump’s VFD, wanting the pressure to be higher, ramps up the pump, resulting in increased pressure. This then causes the valve to close further in order to prevent pressure from rising, which causes the VFD to ramp up the pump. And so on and so forth. Eventually the VFD will ramp the pump up to full power, possibly running it off its operating curve, creating dangerously high discharge pressure and wasting energy.
Generally speaking, there are two effective approaches to preventing this battle for control.
Approach #1: Hand-off Control
A valve and a VFD simultaneously trying to control the same thing is a bit like too many cooks in the kitchen–and a recipe for disaster. The first approach to coordinating pump control valves and pump VFDs is to hand-off control–or in other words, take turns.
For instance, during pump start-up and shutdown, the control valve manages surges by opening and closing gradually. The VFD holds a fixed frequency until the valve is fully open and stable, at which point it “hands off” control to the VFD for normal operation.
Position feedback (via dual limit switches or transmitters) tells the VFD when the valve has finished its movement, ensuring a smooth transition and communication between the two.
This same principle applies to:
Deep well pump control valves and calm network valves that are not controlling for flow or pressure.
High pressure relief valves that only open when pressure is above normal operating pressure.
Altitude valves fed by pump stations supplying the tank.
Pressure relief/sustaining valves that not only relieve high pressure surging, but also relieve excess flow when the pump is supplying more than demand–particularly when demand is lower than the pump’s minimum frequency.
Approach #2: Control Two Different Variables
While too many cooks in the kitchen can be a bad thing, chefs can and do work well together by doing different things that complement each other–such as a saucier who excels in sauce creation and a pantry chef who specializes in appetizers and cold dishes.
In a similar way, pump control valves and pump VFDs can work together if they each control for different, but complimentary variables.
For example:
The VFD controls flow while a back pressure valve controls pressure to prevent pump cavitation. These complementary variables can be coordinated simultaneously.
The VFD controls discharge pressure while downstream pressure reducing valves manage pressure for individual zones. The pressure reducing valves will allow flow based on demand and the pump will supply flow to maintain pressure supplying the control valves.
The VFD controls for pressure while a valve controls for flow, usually used when there are multiple feeds and the valve is controlling for one to have a maximum flow rate.
Conclusion
Pump control valves and pump VFDs don’t have to be rivals. Working together they can provide more effective, efficient, and resilient pump station performance.
If you’re evaluating a system or designing a new station, give us a call! Our team would be glad to help plan effective coordination between your valves and VFDs. We would love to talk through your application and provide recommended valves and valve-VFD coordination controls.