Feature Valves & actuators
Matching valve type to function M
To make the best choice when selecting a valve for an instrumentation system, users need to determine the required function of the valve, says Michael Adkins, product manager general industrial valves, Swagelok Company
ost valves fulfil one of five primary functions: on-off, flow control, directional flow, over-pressure protection and excess-flow protection. Matching valve type to function is the first and most important step in the selection process.
On-off control
On-off control is the most basic valve function. Valves in this category stop and restart system fluid flow. Primary on-off valves are ball, gate, diaphragm, and bellows valves.
Designed for on-off control, ball valves use quarter-turn actuation which starts or stops flow by position- ing a metallic ball in a straight-through flow path. The ball has a large hole through the center of it. When the hole is lined up with the flow path, it enables flow. When it is turned 90 degrees from the flow path, it stops flow. Ball valves offer quick shutoff and high flow capacity.
The position of the handle provides a quick indication of whether the valve is open or closed and for safety purposes, ball valves are easy to lock out and tag. They are most practical and economical at sizes between 1/4 inch and 2 inches (6-50 mm). Typically used for process control applications, gate valves are com- monly chosen for on-off control, particularly for lines above 2 inches. They are also frequently used as the first valve off the process line for process instrumentation, often in a double block and bleed configuration. Gate valves are typically specified in general industrial applications such
as large process or transmission lines. Some can even be larger than 100 inches (2540 mm). Multiple rotations of the handle raise and lower a sealing mechanism in and out of a straight flow path. Shutoff is gradual. Packing surrounds the stem, pre- venting system media from escaping to atmosphere where the stem meets the valve body. Valves that seal to atmos- phere with metal-to-metal seals are referred to as 'packless' because they do not contain the soft packing mate- rial eg gaskets and O-rings normally found around the stem in other valves. The valve stem is the cylindrical part that connects the handle (or actuation) with the inner mechanism for shutoff, flow control and directional control. Usually, the stem turns and/or moves up and down.
All stem seals or packing are subject to wear which can lead to leakage. Valves with packing must be serviced or replaced at regular intervals, although some types of packing create more effective seals and last longer than others such as the two-piece chevron design.
Contrary to packed valves, diaphragm valves are packless and provide rapid shut-off and precise actuation speeds. In some cases, they may also deliver consistent quantities of process fluid. Typically, diaphragm valves are employed in high-purity applications in the biopharmaceutical and semiconductor industries. They provide the highest cycle life, a prod- uct of the valve's highly engineered anatomy. Each valve contains a thin metal or plastic diaphragm which flexes up and down, creating a leak-
high pressure relief valve specifically designed for flow control applications that require both a high relief pressure and a high flow rate in a compact package has been introduced by Lee Products. Capable of relief pressures up to 5400 psid and flow rates over 19 GPM, the Hi-Q valve is suitable for critical flow applications.
A side inlet flow path helps to minimise the installation boss size and
weight, resulting in a valve which measures 98.5mm length x 23mm diameter and weigh just 200 grams. The Hi-Q valve is available in a range of cracking pressures from 1000 up to 5400 psid and designed for nominal system pressures up to 5000 psi and system peak pressures up to 6750 psi. Each valve is 100% tested and inspected to ensure reliable and consistent performance. Lee Products
T: 0753 886664 14
www.leeproducts.co.uk
Valve handles high relief pressures and high flow A
The stem in needle valves provides precise flow control
tight seal over the inlet. This robust valve is usually small, with the largest orifice - or internal pathway - typically less than 2 inches (50 mm).
Bellows valves are also packless, making them a good choice when the seal to atmosphere is critical and access for maintenance is limited. Frequently specified for the contain- ment area in nuclear power plants, a welded seal divides the lower half of the valve where the system media resides from the upper parts where actuation is initiated. The stem, which is entirely encased in a metal bellows, moves up and down (without rotat- ing), sealing over the inlet.
Bellows valves and diaphragm valves are said to have a globe-like flow path. In globe valves, fluid does not flow straight through on a level plane as it does in a ball valve. The flow path enters the valve under the seat and exits above it. Globe valves will have lower flow rates than those with a straight-through flow path of the same orifice size.
Flow-control
Flow-control valves enable the opera- tor to increase or decrease flow by rotating the handle. The operator can adjust the valve to a desired flow rate which it will hold reliably. Some flow- control valves also provide very reli- able shut-off but many turns of the handle are necessary to move from the fully open to the fully closed position. The most common flow-control valves are needle, fine metering, quarter-turn plug and rising plug.
Enter 205
Needle valves provide excellent flow control and, depending on design, leak-tight shutoff. They consist of a long stem with a highly
AUGUST 2011 Process & Control
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44