Do you know all the key service factors an engineer has to consider when specifying a control valve? Every process control system has its own unique set of requirements to keep its critical processes running. These requirements are based on factors such as a service’s flow rate, temperature, pressure, corrosiveness, leakage thresholds and more.
As part of our “Control Valves 101” blog series, we’re going to explain some of the key service factors that you have to consider when specifying control valves. Along the way, we’ll highlight a few of the things we do differently at Trimteck to ensure our customers achieve optimal performance, even for the toughest applications. Let’s start with flow characteristics—an important specification for any process control system.
Flow characteristics
To explain flow characteristics, we first need to explain the valve coefficient and the valve stroke:
- The valve coefficient (Cv) provides a relative measure of a control valve’s efficiency. It quantifies the rate at which a certain amount of fluid can pass through a valve at a particular pressure drop.
- The valve stroke is the full range of movement of a valve, from fully open to fully closed.
Every valve has a flow characteristic. This describes the relationship between the valve coefficient (Cv) and the valve stroke. As the valve opens, the flow characteristic allows a certain amount of flow through the valve at a particular percentage of the stroke.
Essentially, the flow characteristic allows engineers to precisely determine flow rate of their service by understanding how much fluid will pass through the valve when it’s in certain positions.
The three most common types of flow characteristics are:
- Quick-open
- Linear
- Equal percent
There are two rules of thumb for choosing the right flow characteristic:
- A linear characteristic will provide better control if most of the pressure drop is taken through the valve and the upstream pressure is constant.
- An equal percent characteristic will provide better control if the piping and downstream equipment cause significant resistance to the system.
Rangeability
Some valves can handle a large range of flow rates, whereas others excel in a smaller range. This quality is called rangeability. It refers to the ratio of maximum to minimum flow that can be managed by a control valve after receiving a signal from a controller.
So, high rangeability means a valve can control both large and small flows.
Rangeability is affected by three factors:
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Valve geometry - The design of the valve body itself and the regulating element will provide an inherent level of rangeability.
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Seat leakage - Every control valve has a seat in which the valve ‘sits’ when in its fully closed position. It might surprise you to know that not every control valve needs a fluid-tight seat that completely shuts off flow. But, in other applications this is critical. Excessive seat leakage can cause instability as the valve lifts off the seat, which affects rangeability.
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Actuator - Actuators automate the movement of control valves. They affect rangeability because their stiffness influences the stability of the control valve’s movement in and out of its seat. If a control valve’s movement in and out of its seat isn’t stable and smooth, it limits the control valve’s ability to control smaller flow rates.
Diaphragm actuators are commonly used because they are relatively inexpensive, but their accuracy is lower at less than 5% of the valve opening. On the other hand, we know that piston-cylinder actuators can provide control within 1% of valve lift due to the presence of air in their two chambers.
Shut-off requirements
In many process control systems, there will be process stages or events that require you to stop the flow of your service altogether. The degree of accuracy with which you need your control valve to do this is called the shut-off requirement. It’s based on the maximum amount of fluid leakage that is acceptable when the control valve closes.
Safety systems usually have the highest shut-off requirements, especially in industries such as power generation and aerospace and defense. This is because even marginal leakage can have life-threatening consequences.
But, shut-off requirements are still important for process control systems that need to maintain a high level of quality and efficiency. In food and beverage processing, for example, there will be standards for ingredient measures, heating, and cooling. These need to be maintained to minimize wastage and ensure products adhere to regulations and meet consumer expectations.
There are six classes of leakage class which indicate a control valve’s shut-off capabilities. The lower the class, the tighter the shut-off and the less leakage there will be.
Pressure classes
Each control valve is designed to handle a range of pressures up to a certain limit. This is called the valve’s pressure rating. The higher the rating, the thicker the walls of the valve vessel are to prevent it from rupturing when handling a high-pressure service.
Pressure classes also factor in temperature. The higher the process temperature, the less pressure can be handled by all the components in the valve body when it’s fully assembled. This is referred to as the valve body subassembly.
The American National Standards Institute (ANSI) pressure rating system is the most widely used across the world. There are 7 standard pressure classes, with the highest number indicating the highest pressure handling capabilities:
- 150
- 300
- 600
- 900
- 1500
- 2500
- 4500
Simplifying the specification process
Even a top-level look at the key service factors for control valves contains an overwhelming amount of information. That’s before you get to the unique requirements of your own process control system. But, it’s crucial to identify the optimal solution for your application to enhance performance, safety, and product quality.
That’s why the Trimteck team is ready with an expert helping hand. All you need to start a quote with us are your process conditions and existing valve documentation and photos. After you submit them using our online form, one of our Trimteck Applications Engineers will size and specify the most appropriate control valve(s) for your service. It’s that simple.
