Factors To Consider When Specifying Valves
Ask Andy Ryan, the Technical Sales Supervisor at <%=company%>, what the toughest job is that his department faces, and the answer may surprise you. Day in and day out, the biggest hurdle is the lack of industry awareness about valve pressure and flow issues and how they affect a piping system.
"We talk about Cv factors, pressure drop, back pressure and things like that in very specific terms...people often stop us and ask to explain further," says Ryan. "We assume that our customers are familiar with these issues, but the average industry person has so much on their plate these days that they can't do as much research as they would like."
Although Ryan and his staff are happy to talk callers through the process, he has put together a refresher course in a question/answer format for interested readers and especially our distributor salespeople.Plast-O-Matic Valves, Inc.AME=ten>Part I: Ten Frequently Asked Questions... (back to top)
- What does CV factor mean? The definition of CV factor is the number of U.S. gallons per minute of water that will pass through a valve with a pressure drop of one (1) psi. This "factor" can be determined by physically counting the number of gallons that pass through a valve with one (1) psi applied pressure to the valve inlet and zero (0) pressure at the outlet. The maximum CV is a mathematical constant that is determined at the full open position of a valve. For a pressure drop other than one psi, use the equations in number 10 below.
- Does every valve have a constant CV factor? No, a constant CV factor typically applies to full open/full closed shut-off valves.
- Which valves do not have a constant CV factor? Varying CV factors are typically found in modulating or regulating valves that are designed and/or programmed to open to a percentage of full open. Spring loaded valves that are opened by the liquid line pressure do not have a constant CV either; however, the focus of this article is on constant CV applications.
- What is DP? The commonly used symbol DP refers to the pressure drop across a piping component such as a valve or filter. D (the Greek Delta) is the "change" in something; in this case a change, or drop, in pressure. To determine the DP across a valve at a specific flow rate simply subtract the outlet pressure P2 from the inlet pressure P1. The equation is P1 - P2 = DP.
- Why is pressure drop important? Pressure drop is a critical element in system planning and in valve sizing and valve application. The total pressure available in a system and the pressure drops of all valves, equipment and fittings must be known by the engineer designing the system to ensure proper function. If a valve is specified that has too high a pressure drop, the desired flow rate is not likely to be reached. In this situation, the CV factor of the valve is too low.
- What factors determine pressure drop across a valve? The critical factors are the rate of flow, the orifice size and internal flow path. Typically, the lowest pressure drop is across a full open ball valve due to its straight through flow; whereby, a higher pressure drop is realized with a diaphragm valve due to the changes of direction as liquid passes through the valve.
- What is back pressure? Back pressure is simply defined as the pressure found at the outlet or "back" of a valve. It can be caused by downstream piping restrictions resisting the liquid flow or by a separate pressure source such as a pressurized tank or pipeline that is being fed by the valve.
- What restrictions downstream of the valve create back pressure and why is it important to know? During flow conditions components such as fittings, spray nozzles, filters, excessive piping length, reduction in pipe size and vertical rise all contribute to back pressure. It is important to know the maximum potential back pressure in a piping system when installing or specifying a valve. If the maximum back pressure rating of a valve is exceeded, it will not be able to close and stop the flow of liquid when desired.
- What is the relationship between the flow rate (GPM) and pressure drop (DP)? The flow rate in a system determines pressure drop. The higher the flow rate through a restriction such as a valve, the greater the pressure drop across the valve. Conversely, the lower the flow rate the lower the pressure drop.
- How do the Cv factor, GPM and DP work together to size a valve? At least two of these elements are necessary to properly specify a valve. Here are the flow equations: where G = Specific Gravity of the liquid
Now Ryan says, "Getting salespeople to understand the basics in Part I is not difficult. It is simply a matter of their reading through it and digesting it; however, reality sets in quickly when they try to apply the newly learned information in an actual application."
Ryan adds, "It is the exception rather than the rule when salespeople are provided with all of the necessary data regarding their customers' flow systems. Typically, they are frustrated by a lack of specific and reliable information."Plast-O-Matic Valves, Inc.AME=the2>Part III: The Solution (back to top)
Ryan explains the best way to avoid the common pitfalls: "Usually, the wisest approach is to determine who planned or designed the flow system and to secure the necessary information from that person. Getting it second or third hand is not as dependable a method.
The necessary information must include the maximum as well as minimum pressure that will be supplied to the valve inlet, the media, specific gravity and temperature; the flow rate required to pass through the valve and the nature of the piping and restrictions downstream of the valve. If the system designer knows the CV flow factor required for the valve, the selection is made that much simpler. With all of the above information, we can avoid the costly mistakes of improper valve selection, and consequently, keep the customer satisfied."