Our Frequently Asked Questions attempts to provide answers to the most commonly asked questions relating to our products and services. If you don’t find answers to your questions here, please get in touch with your nearest Stewarts & Lloyds branch.
What is a Valve?
A valve is a device that controls the flow of a fluid or gas.
Today's valves can control not only the flow, but the rate, the volume, the pressure or the direction of liquids, gasses or slurries through a pipeline.
What is a pressure relief valve?
Relief valves are automatic valves used on system lines and equipment to prevent over pressurization. Most relief valves simply lift (open) at a preset pressure and reset (shut) when pressure drops only slightly below the lifting pressure.
System pressure simply acts under the valve disk at the inlet of the valve.
When system pressure exceeds the force exerted by the valve spring, the valve disk lifts off its seat, allowing some of the system fluid to escape through the valve outlet until system pressure is reduced to just below the relief set point of the valve. The spring then reseats the valve.
What is a ball valve?
Ball valves, as the name implies, are stop valves that use a ball to stop or start the flow of fluid. The ball performs the same function as the disk in the globe valve.
When the valve handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet.
When the valve is shut, which requires only a 90 degree rotation of the handwheel for most valves, the ball is rotated so the hole is perpendicular to the flow openings of the valve body, and flow is stopped.
What is a check valve?
Check valves are used to allow fluid in system in only one direction. They are operated by the flow fluid in piping. A check valve may be the swing type, lift type, or ball type.
Can butterfly valves be used for flow control or throttling?
This is generally impractical due to the very unsteady nature of flow through a partially-opened butterfly. Variations of the basic butterfly design do not significantly improve this characteristic.
The butterfly valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle.
To open or close a butterfly valve turn the handle only one quarter turn to rotate the disk 90 degrees. Some larger butterfly valves may have a handwheel that operates through a gearing arrangement to operate the valve.
This method is used especially where space limitation prevents the use of a long handle.
Where can I use gate valves?
Gate valves are used when a straight-line flow of fluid and minimum restriction is desired.
Gate valves are so named because the part that either stops or allows flow through the valve acts somewhat like the opening or closing of a gate and is called, appropriately, the gate.
The gate is usually wedge shaped. When the valve is wide open, the gate is fully drawn up into the valve, leaving an opening for flow through the valve the same size as the pipe in which the valve is installed. Therefore, there is little pressure drop or flow restriction through the valve.
A gate valve is usually used as a shut-off valve, gate valves are not suitable for throttling purposes since the control of flow would be difficult due to valve design and since flow of fluid slapping against a partially open gate can cause extensive damage to the valve.
Gate valves are classified as either RISINGSTEM or NONRISINGSTEM valves.
On the nonrising-stem gate valve the stem is threaded on the lower end into the gate. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads, while the stem remains vertically stationary.
The rising-stem gate valve has the stem attached to the gate; the gate and stem rise and lower together as the valve is operated.
What is a globe valve?
A Globe Valve is a type of valve for regulating flow in a pipeline, consisting of a moveable disk-type element and a stationary ring seat in a globular (spherical) body.
Globe valves are named for their spherical body shape with the two halves of the body being separated by an internal baffle. This has an opening that forms a seat on to which the movable disc shut the valve.
Globe valves are used for applications require throttling and frequent operation.
What valves are stop valves?
Stop valves are used to shut off or, in some cases, partially shut off the flow of fluid.
Stop valves are controlled by the movement of the valve stem.
Stop valves can be divided into four general categories: globe, gate, butterfly, and ball valves.
Needle valves may also be considered stop valves.
What valve types are available on the market?
Although many different types of valves are used to control the flow of fluids, the basic valve types can be divided into two general groups: stop valves and check valves.
Besides the basic types of valves, many special valves, which cannot really be classified as either stop or check valves, are found in the engineering spaces.
Many of these valves serve to control the pressure of fluids and are known as pressure-control valves.
What is a Three-Way Valve?
Three-Way valves has three ports. Depending on the particular valve, all three port may be open, two ports may be open, or all ports may be closed.
Can you please explain the difference between normally closed valve and normally open valve?
Valves stays closed in de-energized state; opens when energized.
Valves stays open in de-energized state; closes when energized.
Should you choose Forged or Cast valves?
It depends on the application.Many valve applications in pharmaceutical and biotechnology manufacturing require forged valves because of the low ferrite content and better surface finish.
Cast valves can sometimes have surface imperfections where bacteria can grow and cause contamination.