In the second installment from Swagelok, Michael Adkins product manager, general industrial valves, looks at directional flow valves, overpressure protection valves and excess flow valves, and explains what you need to consider
Part one of this article, in Process & Control August 2011 p14-15, discussed how matching valve type to function is the first and most important step when selecting a valve for an instrumentation system.
Given the seemingly overwhelming number of choices in valve type, size, configuration, materials of construction, and actuation modes, you will make the best choice by asking the first question in valve selection: What do I want the valve to do?
Part one examined two primary valve functions – on-off control and flow control, and reviewed the basic types of valves for each function.
Part two of the article, below, looks at the other three functions and valve types for each: directional flow, over-pressure protection, and excess-flow protection.
Directional Flow Valves
This type of valve directs fluid flow.
Check valves (see Figure 4) ensure flow in one direction only. In most designs, the upstream fluid force pushes a spring-loaded poppet open, allowing flow. In the case of an increase in downstream or back-pressure force, the poppet is forced back into the seat, stopping reverse flow. Check valves are available with fixed or adjustable cracking pressures.
Some ball valves and diaphragm valves are designed with multiple ports. In most multi-port valves, fluid enters through a single inlet but may exit through one of many outlets, depending on the position of the actuator. Multi-port valves may or may not have a shutoff position (see Figure 5).
Overpressure Protection Valves
Valves in this category prevent the build-up of system pressure beyond a certain pressure setting. They are available in two types: relief valves and rupture discs.
One type of relief valve is a proportional relief valve (see Figure 6). It contains a vent to atmosphere that opens when pressure in a system exceeds a certain point set by the operator. A spring-loaded poppet enables the measured release of fluid. The vent closes when pressure returns to a point below where it was set.
A safety relief valve is designed to open quickly, releasing a large amount of system media. Because of their critical safety function, safety relief valves are required by code in certain applications. Safety relief and proportional relief valves are not to be used interchangeably with check valves, since the three have different functions.
Rupture discs are used mainly on sample cylinders to protect against over-pressurization, which may occur, for example, when temperatures rise during transport. Similar to relief valves, rupture discs vent to atmosphere. A metal diaphragm bursts when pressure reaches a set point. This value is preset by the manufacturer. Once activated, the rupture disc must be replaced. Transportation codes require that compressed gas cylinders be equipped with a pressure relief device. A rupture disc is an economical choice for this application.
Excess Flow Valves
Excess flow valves stop uncontrolled release of system media if a downstream line ruptures. Under normal conditions, a spring holds a poppet in the open position. In an excess flow condition downstream, the poppet moves to a tripped position stopping almost all the fluid flow. When the system is corrected, the valve returns to its open position. These valves are available with fixed tripping values.
Once you have matched valve type to function, you are well on your way in the valve selection process. Many details remain, though. You will need to give detailed attention to each of the following, if you haven’t done so far:
• Installation issues, maintenance schedules and access • Safety and code requirements • System parameters, such as pressure, temperature, flow rates, and system media.
You will need to determine:
•Valve size and actuation types; and
•Materials of construction (including O-rings and seals), which must be compatible with the chemical composition of the system media, pressures, and temperatures.
The manufacturer’s representative will be your guide in this process. Product catalogues and test reports are also valuable resources.
Valve selection – Tips and Traps
Know your application. When choosing a valve, you must have certain pieces of information in hand, including the chemical composition of the system media and the full range of pressure and temperatures over the course of the valve’s life. Make sure your valve choice can accommodate these parameters. Don’t go with hunches or approximations. Consult the product data.
Check for material compatibility. It is possible to have the right valve but the wrong materials of construction. Valves will often come with a standard set of materials, but there are alternatives. You should always check the product catalogue to identify temperature and pressure ranges, as well as compatibility with different system media (chemicals). When in doubt, consult your manufacturer’s representative.
Know your maintenance schedule. Different valves have different maintenance schedules, and your system parameters, including the number of times the valve is cycled, will affect this schedule. The valve’s maintenance schedule needs to be manageable for your maintenance team. This seems like an obvious point but it is often overlooked. Are you willing to service that valve once every 20 days when it is 100 feet in the air?
Understand pressure drops. Valves or other components produce a drop in pressure. You need to be aware of the cumulative pressure drop because otherwise you may end up with too little pressure at a certain point in the line. Every valve is rated with a flow coefficient (Cv), which describes the relationship between the pressure drop across an orifice, valve, or other assembly, and the corresponding flow rate. The higher the Cv, the lower the pressure drop. A ball valve and needle valve of the same size will produce very different pressure drops. A ball valve will produce very little pressure drop, whereas a needle valve (or other globe valve) will produce a significant pressure drop.
Consider cost of ownership. The true cost of a valve is not its purchase price, it’s the purchase price plus the cost of owning and maintaining or replacing that valve over time. To calculate the cost of ownership, you must know how long a valve will operate in your particular system between maintenance checks. Maintenance costs must be figured not only in replacement parts, but also in labour and downtime. Note that some valves are much easier to service than others. Some can be serviced in place; others must be removed from the process line. Also, given your valve choice, what are the chances of unscheduled maintenance and downtime?