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| Home | Valves
and applications
Valves are a common sight in our petroleum industry. In principle, they’re not much different from the tap you close after you brush your teeth in the morning. In practice, they are a great leap in quality and design, each with their own merits. They can reduce a flow or even completely stop the flow of a gas or liquid. Different applications require different designs, and each design has its strong and weak points. Some work best when they’re usually open, others are better left closed; others yet are better at throttling, or reducing flows across them. Others yet are designed to only flow in one direction and automatically close if flow changes direction. Valves can be operated remotely by a motor, diaphraghm, or by hand wheel, or directly via a lever mounted to the valve. The one thing all valves have in common is that something is placed in the way of the flow across them. Sometimes this is a flat piece of steel will suffice, but others are of more intricate design with a seat, and a machined-to-match plug of some sort. The Gate Valve usually just slides a "gate" in the way of the flow. Such a valve does not cause much of a pressure drop across it, and are well suited to being fully open, or fully closed. You would typically see such a valve on the side of a tank at a battery. The Globe Valve uses the above mentioned machined surfaces, but reduces the area through which the fluid or gas flows. When closed, a tapered plug is seated inside of a machined, also tapered, hole. When opening the valve, the plug is slowly raised out of the seat and thus allows flow across it. These valves are well suited to controlling flow. As the seat and plug are usually a lot smaller than the diameter of the pipe, and also because the process inside is forced to change direction a number of times, there is a greater pressure drop across such a valve. These types of valves are often seen on pressure vessels such as separator packages. When attached to an actuator of some sort, they can be used to maintain a constant pressure on a vessel’s inlet stream. They can also be used to shut in a vessel if the upstream pressure is too high. It can thus serve as a safety measure also. The Ball valve is seen in a number of applications. They are used for uses where flow is either closed or open, like in a gate valve, but with the following exception: These can also handle a great pressure differential across them. Ball valves are much like a ball with a hole drilled in them. The ball gets a quarter turn and allows the full flow across it. The large area across which the pressure is distributed inside the body allows it to be opened and closes with ease. Because of this ease of operation, ball valves also find occasional use for throttling flows. Ball valves are commonly seen on separators and used as block valves to isolate the vessel from its upstream and downstream stable mates. Block valves are commonly put in upstream of items that require regular maintenance such as dump valves on treaters or separators. Check valves are valves, which permit one way flow only. These can be used to ensure that your treater can only dump liquid to a tank, and not vice versa. These are also used on wellheads. Designs range form a simple spring-loaded flap to a ball in a seat. There are all kinds of connections to decide on when ordering valves. For example you can weld your valve in; clamp it in, like in the case of Victaulic couplings; bolt it in, such as is the case in flanged connections; or you can screw it into place. Usually screwing a valve in is easy during the manufacturing stages, but leaves operators with a lot of work when they need to replace an in-line valve. Victaulic couplings were commonly used in the past. They use a set of "c" clamps and some seals to join two pieces. "Vic" couplings are seen less often now, but are making a comeback as people realize their ease of use as well as their flexibility to angle the valve at will. Flanged connections are more commonly found in the field now, and appear to be the preferred way to go on most production equipment. A flanged valve can easily be removed, and has the added advantage that in cases where vessels or piping is internally coated, damage to coating is minimized during removal/installation. The type of connection on a valve makes a difference, but material choices can also affect performance and usefulness in the field. Three common materials found in our industry. In order of price, they are cast iron, ductile iron, and forged steel. Cast iron is less expensive, and works well. A simple casting is machined to fit its internals and usually delivers good performance. Ductile iron is marginally more expensive, but offers the advantage that it typically weeps prior to failure. As a result, a failing ductile iron valve will "weep" oil for a while and thus alert the operator that replacement or repair is imminent. Cast iron is not as friendly. When it fails it typically breaks. This is what engineers call a "catastrophic" failure. The end result is that a valve will just crack and start spewing $ 29.00 US/Bbl oil all over your lease site. Advantage: Ductile Iron. The third material, Forged Steel is much more expensive, but offers another significant step up in terms of reliability. As tooling costs of forged equipment is significantly higher, the price goes up accordingly. Typically, this type of material is found on higher pressure, and environments that are more corrosive. Pricing of valves can range drastically between two valves apparently designed to do the same job. The difference is often not in the job each valve is capable of performing, but rather in the way the body and parts are constructed. Well-made equipment is machined such that components can be replaced and serviced, whereas others are a complete throw away when they fail. This should be a consideration when buying valves for specific types of service. You may find that a more serviceable valve in a high usage area provides better long-term economics than a cheaper valve that requires frequent overall replacement. |
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