An Engineer’s Guide to Fluid Valve Types & Material Selection

Datetime： 2017/9/13 13:41:42  Hits： 4636

Selecting the correct fluid valve type and material of construction is critical for the safety, quality, yield and process control. There are huge varieties of valve types and valve material and the task of proper selection can be overwhelming. In this article, we will try to understand the fluid valve types, functioning, applications and material of construction which will make the selection task easier.

What are fluid valves?

Fluid valves are mechanical devices that control the rate of flow and pressure of the fluid within the system by opening, closing or partially obstructing the flow of the fluid. Fluid valves are critical components of a piping system that handle liquids, slurries, etc. and can also regulate and direct the flow of the fluid.

Basic functions of fluid valves

•  Starting and stopping of flow
•  Increase or reduction of flow
•  Regulation of the flow pressure
•  Controlling the flow direction

Types of fluid valves

There are different types of fluid valves like ball valve, gate valve, globe valve, butterfly valve, plug valve, needle valve, diaphragm valve, check valve, pressure relief valve, etc. Selection of the proper fluid valve depends on the application and the fluid being handled.

(1) Ball valve

A ball valve is a quarter-turn rotational motion valve that uses a ball shaped disk to start and stop the flow. On opening the valve, the ball rotates to a point where the hole in the ball is in line with the valve inlet-outlet which allows the flow of the fluid. If the valve is closed, the ball is rotated so that the hole is hidden from the valve inlet-outlet resulting in the stoppage of fluid flow.

(2) Gate valve

Gate valves are one of the most commonly used valves in industrial applications. Gate valves are linear motion valves in which a flat closure element slides perpendicularly into the flow of the stream to provide shut-off of the flow. Gate valves are primarily used for starting and stopping the flow of the fluid. 

(3) Globe valve

Globe valves are linear motion valves and are primarily used to start, stop and regulate flow of fluid. The plug or the disc of the globe valve proportionately opens and closes the valve body opening to actuate the start-stop mechanism and the flow regulation. 

(4) Butterfly valve

A butterfly valve is a quarter-turn rotational valve, which is used to start-stop and regulate the fluid flow. A butterfly valve has a disc which is mounted on a rotating shaft. When the butterfly valve is fully closed, the disk completely blocks the fluid flow line. And when fully open, the disc is perpendicular to the flow of the fluid.

(5) Plug Valve

Plug valves are valves with cylindrical or conically tapered plugs which can be rotated inside the valve body to control the flow through the valve. The plugs in the plug valves have one or more hollow passageways going sideways through the plug, so that fluid can flow through the plug when the valve is open and vice versa.

(6) Needle valve

Needle valves have a slender, tapered point at the end of the valve stem that is lowered through the seat to restrict or block flow. Fluid flowing through the needle valve turns 90 degrees and passes through an orifice that is the seat for a rod with a conical shaped tip. Needle valves are used to precisely and gradually control fluid flow in delicate gauges and instruments.

(7) Diaphragm valve

A diaphragm valve is a linear motion valve that is used to start, regulate, and stop fluid flow. The name is derived from its flexible disk, which mates with a seat located in the open area at the top of the valve body to form a seal. The diaphragm is connected to a compressor and the diaphragm moves according to the movement of the compressor.

(8) Check valve

Check valves are spring loaded, non-return valves that provide free flow in only one direction but prevent flow in the opposite direction. Check valves can be installed in any orientation. Check valves are used to stop backflow and protecting equipment which require unidirectional flow to avoid sudden backflow surges.

(9) Pressure relief valve

Pressure relief valves are used to maintain pre-determined pressure of the fluid system. Pressure is set through the adjusting screw which is directly connected to the spring. When the pressure of the fluid system exceeds the pre-determined limit, the valve opens to bypass the fluid to ensure that the pressure drops to the limit set.

Valve type selection matrix 

We have seen basic introduction to different types of fluid valves. The below tabular matrix is a broad indication for the selection of valve type based on the purpose and application. Final selection of the valve type is based on actual application more detailed understanding of the fluid system.

NO	Valve Type	Flow path	Flow control	Applications
1	Ball valve	Straight	Poor	Can be used for clean gas, compressed air and liquid service.
2	Gate valve	Straight	Average	Slurry service, heavy oils, light grease, varnish, molasses, honey, cream, etc.
3	Globe valve	Globe	Good	Cooling water systems, fuel oil, feed water, chemical feed, lubrication oil, etc.
4	Butterfly valve	Straight	Average	Slurry service, vacuum service, high pressure & temperature water & steam services.
5	Plug valve	Straight	Good	Steam, water, oil, gas and chemical liquid service.
6	Needle valve	Globe	Excellent	Instrumentation control, line shut-off and pressure regulating.
7	Diaphragm valve	Globe	Good	Flow control in chemical, mining and water treatment industries
8	Check valve	Linear	Poor	Firefighting, flood prevention, power plants, boiler systems, petroleum refining, etc.
9	Pressure relief valve	Angular	Poor	Boilers, pressure vessels, compressors, refineries, cryogenic, pump stations, etc.

Importance of selecting the right valve material

What material should be the fluid valve made up of? Prima facie, this question might seem trivial, since for many valve is a valve irrespective of its material of construction. But this question is equally important like the selection of the correct fluid valve type. Imagine using a brass valve in food processing industry which might lead to lead poisoning in the food products. And for example using a plastic valve in a high temperature and high pressure environment which might lead to leakage and bursting. Hence it’s very important for an engineer to select the correct valve material based on the purpose, process, type of fluid to be handled and application.

Types of valve material

(1) Cast Iron

Cast Iron is mainly used for comparatively low pressure and low temperature applications. Cast iron has lesser tensile strength and shock resistance than Steel. But Cast iron has excellent fluidity and can be cast into intricate and complex shapes and is an economical material.

(2) Ductile Cast Iron

This is commonly used in steam, water, gas and oil pipeline systems because of its wide range of working temperature and pressures. Ductile Cast Iron offers high performance in comparison to its relatively lower cost than Steel.

(3) Bronze

Bronze is an alloy of copper, tin, zinc and lead. Bronze has good wear resistance, corrosion resistance and machinability which allows it to be shaped into complex castings. Bronze is suitable for valves used in low/medium pressure service. It has a strong resistance against saltwater corrosion.

(4) Brass

Brass is an alloy of copper and zinc. Brass has excellent forgeability and machinability and is more economical than Bronze. Though Brass can go through a process called dezincification, in which the zinc in the part seeps out over time, leaving the copper superstructure which can become brittle and cause failure.

(5) Cast Iron lining

Nylon 11 and epoxy resin are powder-coated on the inner and outer surfaces of iron valve, creating a lining that is useful for preventing red, rusty water. It is also suitable for use in drinking water supply lines.

(6) Cast Steel

Because of its wide range of working temperatures and pressures, Cast Steel is commonly used for all pipelines in oil fields, refineries and petrochemical plants. Mechanical properties of Cast Steel are better than Cast Iron but casting properties are worse.

(7) Special Alloy Steel

Special Alloy Steel has extreme temperature stability and has the highest resistance to corrosion and pitting corrosion. In comparison to Carbon Steel, it has higher strength, hardness, toughness, wear resistance and hot hardness.

(8) Stainless Steel

Because of its excellent corrosion resistance and durability, Stainless Steel is commonly used in a wide range of applications ranging from the petrochemical industry to general plant systems. Stainless steel is unique for its higher cryogenic toughness, attractive appearance and lower maintenance.

(9) PVC & Other Plastics

Plastics don’t corrode like metal valves, making them a leading material in some industries. Although there is a wide range of plastics available, PVC is the most commonly used which is resistant to water and chemicals including acids. However plastics have lesser resistance to high temperature and high pressure in comparison to its metal counterparts.

SRL	Valve material	High temperature resistance	High pressure resistance	Corrosion resistance	Strength & Durability	Applications
1	Cast Iron	M	M	M	M	Water service, HVAC, steam, gas and other general utility
2	Ductile Cast Iron	H	H	H	H	Steam, oil and gas, limited chemical, petroleum refining and production.
3	Bronze	M	M	H	H	Water service, HVAC, low pressure steam, compressed air, LP gas, etc.
4	Brass	M	M	H	M	Fuel line shut-off, air service lines, water service lines, plumbing, etc.
5	Cast Iron lining	M	M	H	H	Drinking water supply lines, mineral acids, salts in water & slurry duties
6	Cast Steel	H	H	H	H	Cryogenic, oil & gas production, refining and petrochemical, etc.
7	Special Alloy Steel	E	E	E	E	Chemical industries, pharmaceutical processing, food & dye
8	Stainless Steel	H	H	E	H	Heat exchangers, chemical vessels, pipelines & food industry
9	PVC & Other Plastics	L	L	E	H	Water distribution, wastewater treatment, chemical processing, etc.

** Legend – E = Excellent, H = High, M= Medium, L = Low, P = Poor **

Bottom-line for Engineers

Engineers first need to study and ascertain the purpose and location of the fluid valve. Based on this, the type of fluid valve needs to be decided. Next comes the careful consideration of the physical and chemical properties of the fluid to be handled and the process parameters and conditions of the fluid system. Based on these factors, the material of construction of the fluid valve needs to be finalized. There are other few factors like cost, service life, ease of operation and maintenance which also need to be considered before finalizing the type and material of construction of fluid valve.