Valves, in instrumentation and control loops, are also known as the final element of control.
CONTENTS
What is a valve?
From one author to another, different definitions arise in the literature. Here, a simple, yet hopefully complete, concept of valve is presented,
A valve is a device/component thought for flow control in a pipe or conduct.
Also, a valve has a number of important features since there are several models of these common devices. For example, a valve can be,
- manual or
- automated.
On the other hand, a valve may be actuated, or closed/open or throttled, in several ways such as,
- electrically,
- pneumatically or
- mechanically,
to say some mechanisms available. Further, valves are also featured by its geometrical internal design so that one may find valve names such as,
- gate valve,
- globe valve,
- diaphragm valve,
- check valve or
- ball valve,
among other specimens.
 |
| Different types of valves |
Valves can be found in many applications and some models may not fit the present definition. However, in this post we are concerned with flow of fluids in closed conduits.
Valves purposes
The vast models of valves is due to the wide variety of applications in industry. Thefore, different designs were needed to satisfy, sometimes, strange needs.
Some examples of industry needs or requirements leading to new valves designs are,
- equipment isolation for maintenance,
- pipe section isolations for maintenance or process shutdown,
- flow modulation (throttiling),
- dosification (small flow rates and throttling),
- unidirectional flow,
- quick closing for safety purposes,
- automatic opening/closing,
- among others.
In this way, every existing valve was originally driven by a need in industry. On the other hand, valves may introduce undesirable effects such as,
- pressure loss,
- source of leakage (if failure occur),
- hammer effect in sudden pipe shutdown,
- for example.
In this way, every valve is to be used for certain application. Here is a list of recommended cases where some valves are recommmended,
- for equipment isolation during maintenance operations only, gate valves are recommended. Since, during equipment operation valve is to be fully open the smallest possible pressure loss is desired too. This also applies for the case of pipe sections isolation,
- for throttling, globe valves are generally recommended. If flow rate is too small or dosification is required, a needle valve may be a good choice. However, pressure losses are increased,
- for unidirectional flow the most common valve is the check valve. There are several designs of check valves but the working principle is barely the same. However, this valve design have the disadvantage of causing high pressure loss,
- for safety purposes, ball valves are recommended since it only requires 3/4 turns to open or close. It also causes a very small pressure loss so that it is a reasonable replacement for gate valves,
- on/off applications may include on/off valves such as the solenoid valve, for example. Since these valves are usually fully open or fully closed, its internal design is that of a gate valve (but not always). Globe or needle design are not common for this type of applications. However, diaphragm valves may also be found in on/off configurations,
- among other situations.
Specification of valves
This is a tak not usually performed by process engineers but by vendors or trustable suppliers. However, some guidelines can be presented.
For short, valve specification can be split into
- valve selection and
- valve dimensioning.
Valve selection refers to systematically pick an option valve design from a set of candidates based on a citeria. Although, it would seem very simple and straight for experienced engineers it would not for novices.
Working conditions such as: temperature, pressure, fluid viscosity, fluid phase and flow rate are required to performed a proper valve selection. In this stage, valve components material and process fluid compatibility is also considered.
Valve dimensioning refers to the calculation of the port size, type of port connection, and internal components of a valve. For simple applications with no complex working conditions dimensioning would be a waste of time but for flow control or throttling it is key and there are in fact dedicated standards and software for this task.
For this calculations, the supplier usually refers to the ISA 75.01 Control valve sizing equations standard.
Check your knowledge on valves generalities
Follow the link below to test what you have learned from this post,
What you know about valves generalities
This is the end of the post. I hope you find it useful.
Other stuff of interest
- LE01 - AC and DC voltage measurement and continuity test
- The pros and cons of bulb thermometers
- The pros and cons of bimetallic thermometers
- Some examples of temperature instruments
- Minor losses - Formulas
- What is a process variable?
- What are the most important process variables?
- Time dependence of process variables
- A list of process variables
- PID controllers for temperature
- Valves - Gate design
Ildebrando.
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