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How much do you know about valves?
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How much do you know about valves?

Views: 73     Author: Site Editor     Publish Time: 2018-01-02      Origin: www.fuchun-casting.com

Valves are usually made of metal or plastic and they have several different parts. The outer part is called the seat and it often has a solid metal outer casing and a soft inner rubber or plastic seal so the valve makes a closure that's absolutely tight. The inner part of the valve, which opens and closes, is called the body and fits into the seat when the valve is closed. There's also some form of mechanism for opening and closing the valve—either a manual lever or wheel (as in a faucet or a stop cock) or an automated mechanism (as in a car engine or steam engine).

It's often critically important for valves that are switched off to allow absolutely no escape of liquid or gas through a pipe to avoid accidents, explosions, pollution, or the loss of valuable chemicals (even a dripping faucet can be expensive if your water is metered). That's why the seal on a valve needs to be perfectly secure and a valve that's turned off must be tightly closed. Turning off a high-pressure flow of liquid or gas by obstructing it with a valve is physically hard work: in other words, you need to use a lot of force to do it. That's why some valves are operated by long levers (as in our top photo) or large wheels (as in the photo shown here). If really big valves require too much force for a human to supply, they're operated by hydraulic rams.



   ● Stopping and starting flow;

   ● Reduce or increase a flow;

   ● Controlling the direction of flow;

   ● Regulating a flow or process pressure;

   ● Relieve a pipe system of a certain pressure.

The many different types of valves all have different names. The most common ones are the butterfly, cock or plug, gate, globe, needle, poppet, and spool:

Ball: In a ball valve, a hollowed-out sphere (the ball) sits tightly inside a pipe, completely blocking the fluid flow. When you turn the handle, it makes the ball swivel through ninety degrees, allowing the fluid to flow through the middle of it.

Butterfly: A butterfly valve is a disk that sits in the middle of a pipe and swivels sideways (to admit fluid) or upright (to block the flow completely).

Cock or plug: In a cock or plug valve, the flow is blocked by a cone-shaped plug that moves aside when you turn a wheel or handle.

Gate or sluice: Gate valves open and close pipes by lowering metal gates across them. Most valves of this kind are designed to be either fully open or fully closed and may not function properly when they are only part-way open. Water supply pipes use valves like this.

Globe: Water faucets (taps) are examples of globe valves. When you turn the handle, you screw a valve upward and this allows pressurized water to flow up through a pipe and out through the spout below. Unlike a gate or sluice, a valve like this can be set to allow more or less fluid through it.

Needle: A needle valve uses a long, sliding needle to regulate fluid flow precisely in machines like car engine carburetors and central-heating systems.

Poppet: The valves in car engine cylinders are poppets. This type of valve is like a lid sitting on top of a pipe. Every so often, the lid lifts up to release or admit liquid or gas.

Spool: Spool valves regulate the flow of fluid in hydraulic systems. Valves like this slide back and forward to make fluid flow in either one direction or another around a circuit of pipes.



  • What is 'multiple certification'?

    This is where a batch of steel meets more than one specification or grade. It is a way of allowing melting shops to produce stainless steel more efficiently by restricting the number of different types of steel. The chemical composition and mechanical properties of the steel can meet more than one grade within the same standard or across a number of standards. This also allows stockholders to minimise stock levels.

    For example, it is common for 1.4401 and 1.4404 (316 and 316L) to be dual certified - that is the carbon content is less than 0.030%. Steel certified to both European and US standards is also common.

  • What surface finishes are available on stainless steels?

    There are many different types of surface finish on stainless steel. Some of these originate from the mill but many are applied later during processing, for example polished, brushed, blasted, etched and coloured finishes.

    The importance of surface finish in determining the corrosion resistance of the stainless steel surface cannot be overemphasised. A rough surface finish can effectively lower the corrosion resistance to that of a lower grade of stainless steel.

  • Can I use stainless steel at high temperatures?

    Various types of stainless steel are used across the whole temperature range from ambient to 1100 deg C. The choice of grade depends on several factors:

    1. Maximum temperature of operation
    2. Time at temperature, cyclic nature of process
    3. Type of atmosphere, oxidising , reducing, sulphidising, carburising.
    4. Strength requirement

    In the European standards, a distinction is made between stainless steels and heat-resisting steels. However, this distinction is often blurred and it is useful to consider them as one range of steels.

    Increasing amounts of Chromium and silicon impart greater oxidation resistance. Increasing amounts of Nickel impart greater carburisation resistance.

  • Can I use stainless steel at low temperatures?

    Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.

    Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.

    Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.

    In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.

    Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite. 

  • Is stainless steel non-magnetic?

    It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1. 

    It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

    In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

    Martensitic, ferritic, duplex and precipitation hardening steels are magnetic.

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