Page 10 - Peter Paul 2019 Catalog

CHOOSING YOUR VALVE
Guidelines for Selecting Valves
There is much to consider when choosing the right valves for an application. Here are some basics, important considerations,and guidelines for energy efficiency.
The two primary characteristics for selecting a directional- control valve are the number of fluid ports and the number of positions the valve can achieve. Valve ports provide a passageway for fluid (liquid or gas) to flow into and out
of the valve. A valve with only an inlet and outlet port is a 2-port (often called a 2-way) valve. A valve with one inlet and two outlet ports— or two inlet and one outlet port — is called a 3-port or 3-way valve. The number of positions refers to the number of distinct flow paths a valve can provide.
Two-position valves operate either on or off, whereas spool valves use a sliding spool to achieve two, three, or more positions. Two-position valves use a plunger, poppet, or ball that seals against a seat, which provides a positive seal so that fluid does not flow when the valve is closed. However, the sliding motion of a spool can cause wear, which compromises the sealing integrity of spool valves. Therefore, a spool valve may pass fluid even if it is closed.
BASICS OF TWO-POSITION VALVES
Two-way, two-position valves can be visualized as a fluid switch; fluid flows when the valve is open, and fluid flow is blocked when the valve is closed. This naming convention
is just the opposite of that used for electrical switches — which pass current when they are closed and block current when they are open. Two-way valves are available for either normally open (NO) or normally closed (NC) operation. A NO valve passes fluid when it is not energized. When the valve is shifted, it closes to block flow. In contrast, a NC valve blocks flow when it is un-energized. Shifting a NC valve opens the valve so it passes flow.
Two-position valves are also available in a 3-way configura- tion. Instead of blocking or passing fluid flow, they pass
fluid through two of their three ports and block flow to a third (usually exhaust) when in one position. When shifted (energized), they unblock the third port and block one of the other two. Two-position valves often are shifted manually, especially when used as a shutoff valve. When physical effort would be too great for manual shifting (and in some special- ized machines), 2-position valves may use compressed air or other pressurized fluid to provide the power
to shift them open or closed. However, for most automated equipment, electricity is used to shift valves from one position to another.
Choosing between NO or NC operation is an important consideration for energy efficiency. For example, if a machine only needs to block flow intermittently, a NO valve should be specified because it will only use electric power periodically. During the majority of machine or system operation, it will pass flow without using any electrical power. However, when used in a safety function, the valve may need to block flow if electrical power is lost. In this case, a NC would be specified because it will block flow if electrical power is lost. A third option is a latching valve, which provides an energy-saving alternative. Latching valves use a permanent magnet to hold them open or closed once they’ve been shifted. This allows them to stay in either state indefinitely without drawing power. A brief DC voltage pulse shifts the valve open, and a reverse pulse polarity pulse shifts it closed.
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