As we know, the definition of friction is “the resistance to motion of one object moving relative to another”. Friction is classified as either static or dynamic. Static friction operates between two surfaces that aren’t moving relative to each other, while dynamic friction acts between objects in motion.
Let’s delve into the details to find out which area of the control valve is subject to friction and how it affects the performance of the valve.
Control valves have moving parts. The valve stem which is in constant contact with stem packing slide over it during the valve operation. The motion of valve stem will be linear in linear valves and rotary in rotary valves. So some degree of friction is inevitable in stem packing.
In some types of valves (e.g. cage guided globe valves and rotary ball valves), trim components must past each other throughout the stem travel. It adds additional friction which is imposed by stem packing.
Static friction is greater than dynamic friction in magnitude. By relating this fact to valve stem motion, we can find that the amount of force required to initiate valve stem (i.e. to overcome static friction) usually exceeds the amount of force required to maintain the valve stem motion (i.e. to overcome dynamic friction). The presence of friction in the control valve increases the force from the actuator required to start and maintain valve stem motion. If it is an electric or hydraulic actuator, the additional force demands additional energy from the actuator. But this case is not the same for the pneumatic actuator. If the actuator is pneumatic, a more series problem arises from the combined effects of static and dynamic friction. Let’s find out how.
Case 1: Opening of an air to open (reverse type actuator) sliding stem control valve with negligible friction.
Imagine an air-to-open sliding-stem control valve with an actuator bench set pressure 0.8 kg/cm2 to 2.4 kg/cm2. The air pressure inside the actuator starts rising from a value of 0. What happens if the air pressure reaches 0.8 kg/cm2? If the actuator spring tension is set properly and there is only a negligible friction present inside the valve, the valve stem starts rising from a fully closed position. As the air pressure increases, the valve stem continues rising smoothly. If we draw a graph of stem motion with respect to actuator pressure, we get almost a straight line.
Case 2: Opening of an air to open (reverse type actuator) sliding stem control valve with substantial friction.
Let’s see what happens if we open a valve with substantial friction from the closed position. Consider the above-illustrated example. We try to open an air-to-open sliding-stem control valve with an actuator bench set pressure 0.8 kg/cm2 to 2.4 kg/cm2. The air pressure inside the actuator starts rising from a value of 0. Imagine that there is substantial friction present in the valve assembly. Does the valve start lifting smoothly if the air pressure inside the actuator increased to 0.8 kg/cm2? No! This valve will remain fully closed until enough extra pressure has accumulated in the actuator to generate a force large enough to overcome spring tension plus valve friction.
As the stem moves, the chamber volume in the diaphragm or piston actuator increases which eventually leads to a pressure drop inside the actuator. As a result, the actuating force which moves the stem decreases. When the force decreases sufficiently, the stem stops moving because the available force is not adequate to overcome the static friction. The stem will remain stationary until the applied pressure increases sufficiently again to overcome static friction, then the “slip-stick” cycle repeats.
If we graph the mechanical response of a pneumatic actuator with substantial stem friction, we see something like this:
Compare this graph with the one we have drawn for the non-friction case. What’s the difference you can find out here? Instead of a smooth straight line, we got a series of “stair-steps” here. The combined effects of static and dynamic friction plus the dynamic effects of a pneumatic actuator make precise valve positioning too challenging or nearly impossible. This effect is commonly known as stiction.
Even worse is the effect friction has on valve position when we reverse the direction of pressure change. Suppose we try to close this air-to-open valve by releasing air pressure in the actuator. Note that the stem will travel downwards, which is the opposite direction of the stem travel in the previous case. Let’s see what will happen.
We begin to release the air pressure in the actuator to bring the stem towards the closed position. Due to static friction (again), the valve will not immediately respond by moving in the closed direction. Instead, it will hold still until enough pressure has been released to diminish actuator force to the point where there is enough unbalanced spring force to overcome static friction in the downward direction.
Once this static friction is overcome, the stem will begin to accelerate downward because (lesser) dynamic friction will have replaced (greater) static friction. As the stem moves, however, air volume inside the actuating diaphragm or piston chamber will decrease, causing the contained air pressure to rise. Once this pressure rises enough that the stem stops moving downward, static friction will again “grab” the stem and hold it still until enough of a pressure change is applied to the actuator to overcome static friction.
What may not be immediately apparent in this second scenario is the amount of pressure change required to cause a reversal in stem motion compared to the amount of pressure change required in the first scenario. To reverse the direction of stem motion, not only does the static friction have to be “relaxed” from the last movement, but additional static friction must be overcome in the opposite direction before the stem can move that way.
We may clarify this concept by applying numerical values to this problem. Let’s assume that we have a control valve with static packing friction of 25 kgf in either direction and an actuator diaphragm diameter with a 12-inch diameter (113.1 square inches of area).
We know the equation: Pressure (P) = Force (F) / Area (A). As per the equation, an applied pressure of 0.221 kg/cm2 will be required to overcome this static friction in either direction. That is to say, to lift the valve stem in the upward direction, we need to apply 0.221 kg/cm2 more pressure than ideally required.
After achieving a new valve position by lifting the stem, we desire to move the valve downward to some new stem position. For this purpose, decreasing the air pressure by 0.221 kg/cm2 to relax the tension on the packing is not sufficient. We must decrease the air pressure by another 0.221 kg/cm2 to overcome packing friction in the downward direction before the valve moves at all from its last position.
What is obvious from this numerical model remains, a pressure reversal of 0.442 kg/cm2 (i.e. twice the equivalent value of packing friction) is required to make the valve reverse its direction of motion. This constitutes “deadband” in the control valve’s action, which degrades control behaviour.
Thus, the effects of friction on a pneumatic control valve actuator may be quantified by subjecting the valve to small reversals in applied actuator pressure and measuring the resulting stem position. The largest increment of actuator pressure reversal resulting in zero stem motion represents the total amount of friction within the valve mechanism.
The following graph shows such a test, plotting actuator pressure over time as well as valve stem position over time. As the actuator pressure is stepped up and down in successively smaller intervals, the control valve’s stem position is seen to respond with less and less motion until it fails to respond at all:
The practical method to diagnose the presence of substantial friction in a control valve:
For pneumatic and hydraulic actuators, actuator force is a direct function of fluid pressure applied to the piston or diaphragm. For electric actuators, actuator force is an indirect function of electric motor current. It may be directly measured using load cells or springs and displacement sensors in the gear mechanism.
Many modern smart valve positioners can monitor the driving force applied by an actuator on a valve stem, and correlate that force against stem motion. Consequently, it is possible to perform highly informative diagnostic tests on a control valve’s mechanical health concerning friction. This type of comparison of actuator air pressure versus stem position is commonly known as valve signature.
Take a look at the sample valve signature shown above. The red line indicates valves response in the opening direction while the blue line indicates valves response in the closing direction. The sharp turns at each end of this graph shows where the valve stem reaches its end positions and cannot move farther despite further changes in actuator pressure. The gap between the plots is proportional to the amount in the friction present in the valve mechanism.
The relatively small magnitude of this offset, as well as its consistency, indicates that packing friction in a valve is “healthy.” The more packing friction a valve experiences, the more vertically-offset the two plots will be.
How to reduce excessive packing friction?
The quantity of packing friction present in a control valve depends on mainly three factors:
- The material of valve packing
- No of packings
- The load applied to the packings
Graphite and PTFE are widely used low-friction materials for compression packing. PTFE is a highly lubricious material but it but is limited by its 200°C temperature rating. For the temperatures beyond 200°C, graphite shall be used.
Lesser the no of packings, there will be less contact area between stem and packing. It will lead to low friction. But reducing no fo packings may facilitate valve leakage. Usually, the manufacturer uses an appropriate no of packings from their knowledge and experience, to balance between leakage and friction.
The case is the same for the load applied to packings. Over tightening of the packing nut eager to prevent leakage is a common source of excessive packing friction. So the tightening should be balanced in such a way that the leakage and friction must be minimum.
Valve manufacturers use their best practices to select suitable material for packing and no of packing. While assembling the valve, they ensure that there is no over-tightening and no leakage through the valve. Customer accepts their advice.
The customer should properly maintain the control valve. Periodic replacement of valve packing is highly recommended. Care must be taken by maintenance personnel while replacing valve packings. Over-tightening of packing nut may lead to excessive friction. Seeking support from experienced valve service providers will be highly beneficial in this regard.