The Truth About Problem Valves

A problem with drift
Drift can be a tricky problem in a hydraulic control system. We will discuss two drift topics, the relatively straightforward constant actuator drift and the more elusive issue of null drift. Actuator drift occurs when a valve is out of null, resulting in a piston moving slowly or drifting when there is no control signal (e.g. when the electrical power is off). In some cases, this drift is desired — such as when null is adjusted so that the piston rod retracts to a safe position upon loss of the control signal.

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Problems arise when the rate of drift is too high or in the wrong direction. For example, with a high drift rate, as much as a 10% control signal to the valve could be required just to compensate for the out-of-null valve. If a 10% control output is required just to hold position, only 90% is left to make the actuator move in the direction opposite the drift. Consequently, the actuator may only get to 90% of full speed in that direction. Therefore, in applications where quick moves are needed, a strongly biased null valve can keep the actuator from reaching the desired full speed.

Adjusting the valve null is as easy as turning a screw on the servovalve or turning a pot on a proportional valve amplifier. With the control signal to the valve set to zero, the screw or pot is adjusted until the actuator stops drifting. Alternatively, with the axis holding position in closed loop, you can adjust the null screw or pot until the control signal to the valve is zero volts. The null can also be compensated for in the motion controller by adjusting the bias or null parameter. Keep in mind, however, that this has the speed-limiting disadvantage mentioned above.

If the motion controller has an integrator term in its closed-loop algorithm, the integrator will automatically compensate for the null while it is in closed-loop control mode. However, misusing the integrator as a null compensator can lead to some unexpected behavior. For example, because the integrator is not applied in open-loop mode, the null will not be corrected during jog moves or any part of a cycle where an open loop move is used. So, it is best to adjust the null at the valve or adjust the bias parameter in the motion controller instead of relying on the closed-loop compensation of the PID algorithm.

Beware of the drift
A varying null condition, called null drift, is a more serious problem. This can be caused by backpressure, flow forces, or valves with non-existent or poor spool control. Null drift requires the controller to be constantly changing the output to the valve to hold a position or maintain a pressure.

This can hurt the performance and repeatability of the position or pressure control, although a high-performance motion controller can compensate for the resulting change if the error isn't too great.

Spool control is important to minimize null drift. A well engineered servo-proportional valve controller has an inner control loop that moves the spool position to correspond to a control signal, Figure 1. Ideally, the spool position would move to a +50% flow position when the controller sends the valve a 50% control signal.

Now assume the controller is sending a 0% control signal to move the spool to the null, or 0%, position. As the spool gets closer to the 0%, position the error becomes small, so the force to correct the error becomes small. This force may not be large enough to overcome real world friction or flow forces so a small null error will remain.

Valves with only a proportional control will not reach the desired location, because there isn't enough force from the spool controller to reduce the error to zero. A PI (proportional with integrator spool) controller has an integrator term that will eventually reduce the error between the desired and actual spool location to zero and minimize null drift.

Valves are available in many different styles, sizes, materials and connections. Choosing the right valve is primarily dependant on the task (control or on/off), the service conditions, the fluid (liquid, gas, combustible, corrosive etc..), and the load characteristics of your application. Below are a few suggestions and thoughts to keep in mind when selecting your valve:

1. Valve size:

The right valve choice means matching the valve's size to the expected flow through the system. The valve flow coefficient (Cv), or valve characteristic, is used to select the proper valve size while maintaining stable flow. The simple equation to calculate Cv is:

Additional resources:
Maximizing Performance: The Power of Proportional Valves

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Q = Flow rate
SG = Specific gravity of fluid
∆P = Pressure drop across valve

A common mistake is calculating too high of a Cv by using the maximum flow rate, which results in selection of an oversized valve. The range of flow (min, max and mean flow rates) should be utilized to properly size the valve. However keep in mind that it is important to choose a valve with a Cv value sufficiently larger than the calculated Cv to help provide expected flow performance.

We have made calculating your CV value easier with our valve calculator. The calculator will also show you the most compatible electric actuator options based on your calculated CV.

2. Valve types:


Below are some common valve types and their general usage, as well as a short description of their distinct advantages & disadvantages:

Valve Type Usage Advantages Disadvantages

Ball Valves

Normally limited to strictly an "on–off" control function (fully open or fully closed positions).

Compatible with Hanbay quarter turn electric actuators. Reliable, cost-effective, require little maintenance, diverse (available in a wide range of materials and sizes), full unrestricted flow. Limited throttling characteristics, difficult to clean which can lead to contamination.

Needle Valves

Used in flow-metering applications, to precisely and consistently control a low flow rate (modulating)

Compatible with Hanbay multi–turn electric actuators. High degree of precision and control, flexibility, resistant to both hot and cold temperatures, able to endure constant high pressure and vibrations. Pressure loss is high, can only be used for low flow rates, seat and needle could get damaged if not operated carefully.

Globe Valves

Used for regulating flow or pressures as well as complete shutoff flow.

Compatible with Hanbay multi–turn electric actuators. Good shutoff capability, shorter stroke (compared to gate valves), can be used in high-pressure systems. High pressure drop (head loss), unidirectional, opening speed is slower, not suitable for clean or sterile applications.

Gate Valves

Used in the fully open or fully closed positions. Not normally used to regulate flow because the flow rate of the fluid is not proportional to the amount that the valve is open.

Compatible with Hanbay quarter turn electric actuators. Small fluid resistance, energy efficient, unobstructed flow (does not decrease pressure), bidirectional. Slow acting, bulky, not drip tight shut off, do not partially open as this will cause damage to seat/disc, low pressure limitations.

Butterfly Valves

Used for regulating flow and in the full open and fully closed position. They perform well in large volume water and slurry applications.

Compatible with Hanbay multi–turn electric actuators. Easy and fast to open, lightweight & reduced space requirements, lower maintenance. Flow adjustment range is small, not suitable for high temperature or high pressure piping systems, and sealing performance is poor compared to ball valves and globe valves.

Plug Valves

On-off control, similar to ball valves.

Compatible with Hanbay quarter turn electric actuators. Long service life, high reliability, and ease of operation. Higher initial cost than ball valves, and no throttling capabilities.

3. Chemical and Environmental Compatibility:

Corrosion Resistance: The valve material needs to be compatible with the gases or liquids flowing through it. Hostile chemical environments will cause unsuitable valve materials to deteriorate due to chemical reactions.

Pressure: Consider the pressure ranges where the valve will be installed. Strength in a valve is it's ability to withstand the internal stresses generated by containing and controlling the fluid under pressure.

Temperature: Operating temperature is important to consider when choosing the right valve material for your application. When temperature is raised, a valves material strength will change, usually becoming softer and losing it's strength.

Environment and Maintenance: What type of environment your valve will be installed in and what this means for the maintenance of the valve. Harsh environments include extreme cold/heat, saline, moisture, acidic/alkaline etc...


Matching your valve with an electric actuator: Multi-turn and quarter turn valve actuators explained here. If you are not sure which valve actuator is the right fit for your application, don't hesitate to contact one of our engineers who would be happy to help you with your requirements.

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