Servo Valve Sleuthing - How to identify hazardous failure in your Servo Valve Systems?

Servo valves are designed with a high degree of accuracy and control and are often installed in systems that require efficiency. Given below is a picture of a control system. Servo valves are a close relative of the proportional valve based on an electrical torque motor. They generally use the feedback between the main and pilot spools to give precise control. Thus, where there is a need for precise control, usually one or more electro-hydraulic servo valves are in use.

To understand this better, let’s talk about the basic anatomy of a servo valve. A typical device (Two-stage servo valve) is shown in figure 2 below. The small pilot spool here is directly connected to the torque motor and the pilot spool moves within the sleeve which is linked to the main spool. The right end of the main spool is always connected to the pilot pressure line. Area (A) is reduced because of the rod in between (as shown in fig). The left side pressure of the spool is handled by the pivot valve. The area restriction is not much here and the valve is designed accordingly (area 2A).

If the same pressure P is applied to both ends, the spool experiences a left force of P × A and a right force of 2P × A, causing a net force of P × A to the right, resulting in a shift of the spool to the right.

If a pressure of P is applied to the right end and 0.5P is applied to the left end, equal and opposite forces of P × A result and the valve spool is stationary.

With a pressure of P on the right end and a pressure less than 0.5P on the left, the net force is to the left and the valve spool moves in that direction.

The pilot valve can thus move the main spool in either direction, in a controlled manner, by varying pressure at the left end of the main spool from zero to full pilot pressure.

In case the electrical control signal causes the pilot spool to shift left, this increases the pressure and enables the main valve to shift toward the right which in turn pushes the sleeve left. When the hole in the pilot sleeve aligns with the land, the main valve stops moving. A change in electrical signal moving the pilot spool to the right reduces pressure at the left end of the main spool by bleeding fluid back to the tank.?

This causes the main spool to move left until, again, the pilot sleeve and pilot lands are aligned. Thus, the main valve spool follows the pilot spool with equal, but opposite movements.

How to quickly identify the failures in the Servo Valve System??

Here are the symptoms to look for:

Coil breakage - When the overcurrent comes from the amplifier board or an overload on the output cause coil to overheat may lead to breakage. At this point, the valve would definitely need service as the coil would need replacement. This symptom is often seen as a slow response or the valve not shifting all the way.

Valve core wear - ?If the servo valve has many sliding and shifting parts, and metal touches the metal, you have the potential for wear which may lead to failure. If you or the operator witness drags or sluggishness in the way the valve is operating, it could mean that the core of the valve is experiencing some wear and needs a rebuild.

Spring wear - Apart from the modern valves, a few valves has centering springs to determine the spool position. The spring may degrade over time depending on how often it is compressed. The reason behind the degradation might be the mechanical stress that the spring is subjected to. For example, the excessive load or movement outside of its design intention.?

Jam of the spool - Jamming of the spool can be due to wear, quality of fluid, or manufacturing defect.? One of the major reasons for the jamming could be grit, plastic, metal shavings, or any other contaminant in the system’s hydraulic system. It depends on the position of the machine or what type of product it produces. To avoid jamming, most of the servo valves have filter screens on the ports which can be cleaned or replaced as and when needed.

Failure of on board electronics (OBE) - ?This is one of the most common failures where the electronic of the servo valve mechanism could compromise due to the presence of contaminants, leakages, or poor maintenance practices. However, electronics can also suffer from heat damage from in the valve heat exchangers. In some cases, common failure components like resistors, capacitors, etc. are prone to failure due to noise, load, heat, or contamination.?

Sealing failure - As seals are usually made up of plastic or rubber, they are prone to get damaged due to contamination, age, or heat. Such failure of seals may lead to pressure loss, electronic contamination, and wear of internal machine parts which ultimately leads to valve performance issues or major failure.

Electrical issues - The incorrect feedback signals from the control section of the application lead to Chattering. It can also be caused by weak coils and if the parameters specific to your valve are lost.???

Contaminants - Another reason for chattering could be dirty or clogged filter screens, a contaminant of the spool housing, or nozzle clogging. This can be caused by a number of conditions mostly related to hydraulic fluid integrity. If the fluid is too hot, it can become thin and not properly disperse and coat internal parts leading to mechanical wear and if too cold, it becomes gummy and loses its viscosity. Also, if proper and suitable fluids are not used, the valve would suffer.

When Turbine Trips and why it is important?

A turbine trip actually indicates fast closure of all steam inlet valves for blocking the flow of steam. The turbine trips when the turbine experience any malfunction. Let’s look at the common causes for a trip -?

• The speed of the turbine shaft increases beyond the specific value (for example, 110%)
• Low vacuum in the condenser
• High Vibrations
• Failure of the lubrication system
• Manual emergency turbine trip

Do you know when solo testing is performed, the level of risk to the operator and equipment is at its greatest? It is very important to have a safe, reliable, fast-acting trip protection system. The system is tested while the turbine is down and again tested at speed with the turbine uncoupled from its load. The designer must take into consideration the instant loss of load while the turbine is in operation. However, the entire protection system—electrical, mechanical, and hydraulic components—must perform perfectly. Remember, every turbine must be treated as the most critical and dangerous piece of equipment in the plant.?

Let’s take an instance from a recent case. Due to high moisture (800 ppm) and TAN (0.35 mg KOH/gm), the trip solenoid valve got stuck. The valve was inoperative due to jamming at the time of the emergency trip. Fortunately, something unusual happened and the trip oil pressure got killed resulting in the closure of the main steam stop valve and control valve. It was a narrow escape from a catastrophic failure and many human lives and equipment were saved.?

Finding the correct cause of failure and eliminating them is the key to preventing machine downtime and keeping your plant safe from any dangerous incidents.