What Is a Check Valve? What Does It Do?

A check valve is a type of valve that allows fluids to flow in one direction only. Although its primary purpose is to prevent backflow, there are many types of check valves. Check valve prices vary according to the material used in manufacturing, size and dimensions. The check valve is also known as a non-return valve or one-way valve.

What Is a Check Valve? What Does It Do?

Check valves are one-way safety valves that ensure fluids passing through an installation move in one direction only and prevent them from flowing back, allowing the system to operate more efficiently. These are simple devices. They perform a vital function by preventing reverse flow. Reverse flow in a system can often damage or cause failure of other equipment connected to the installation, such as valves and pumps.

Even without a failure, it can lead to a loss of system integrity. The operation of the valve, which has a single inlet and outlet, is based on pressure difference. The valve opens when the pressure on the inlet side is greater than the pressure on the outlet side. If the inlet pressure is lower than the outlet pressure, it closes and the fluid is prevented from flowing back. Unlike other valve types that work on an open/close principle, check valves do not require a lever, actuator or manual force. Opening and closing take place entirely automatically.

What Are the Types of Check Valves?

There are several types of check valves depending on the system in which they will be used. The selection should be made according to factors such as the type of fluid, mode of operation, installation direction, maximum pressure, connection type, temperature and closing speed. The main types are:

• Swing check valve
• Wafer check valve
• Spring-loaded check valve
• Tilting disc check valve
• Ball check valve
• Disc check valve
• Dual plate check valve

Although they differ in shape and size, their basic purpose of use is the same.

You can obtain information by visiting the Check Valve page in our product range.

How Does It Work?

Cracking Pressure

For the valve to operate, a minimum pressure is required between the inlet and outlet. The lowest pressure that causes the valve to open is called the cracking pressure. This minimum required pressure varies depending on the size and design of the valve. When selecting a check valve, it should be ensured that the system can generate this pressure.

Closing

If the flow pressure in the system drops below the cracking pressure or a reverse flow occurs, the valve will close. The closing mechanism varies according to the valve type. In all cases, the mechanism inside the valve prevents backflow and ensures tightness.

Installation Direction

Check valves operate in one direction only. It is therefore essential that they are installed in the correct direction. If installed in the opposite direction, the flow stops and pressure builds up. This in turn damages other equipment in the system. Every valve bears a marking that indicates the flow direction.

What Criteria Should Be Considered When Selecting a Check Valve?

There are a number of points to consider when selecting the valve. Some of these criteria are listed below.

• Connection type: flanged, threaded, grooved, etc.
• Maximum pressure and minimum cracking pressure
• Horizontal or vertical installation direction
• Size
• Material compatibility
• Internal and external temperature
• Easy access for repair

Common Mistakes in Check Valve Installation

The most common reason a check valve does not deliver its expected service life is not a failure but incorrect installation. The four situations most frequently encountered in the field are: Installing the flow direction in reverse. Every valve body bears an arrow indicating the flow direction. If the arrow does not face the flow direction of the line, the valve remains permanently closed and the line behaves as if blocked. This is detected immediately by measuring the pressure drop during commissioning. Installation too close to the pump outlet. Turbulence at the pump outlet causes the disc to flutter continuously and the seating surface to wear prematurely. Manufacturers generally recommend leaving a straight run of at least five pipe diameters between the valve and the pump. Using a horizontally designed valve in a vertical line. Some swing-type check valves are designed for horizontal installation only. When fitted in a vertical line, gravity does not close the disc fully and back leakage occurs. The installation direction must be determined at the selection stage. Operating at low flow. The valve does not open fully at flow rates below the cracking pressure. The disc oscillates in a half-open position, producing both vibration and rapid wear. The minimum flow of the line should be above the opening value of the valve.

Water Hammer and the Check Valve

When the pump stops, the flow changes direction and the valve closes. If closing is not completed before the flow fully reverses, the returning liquid is stopped abruptly; this creates a pressure wave known as water hammer. The pressure wave can reach several times the nominal pressure of the line and cause damage to gaskets, weld seams or pump packing. In lines with a high risk of water hammer, spring-assisted (nozzle) or dual plate check valves are preferred. In these types, closing is completed before the flow reverses, so the surge is largely prevented. This choice becomes important in long discharge lines and at pump outlets operating at high static head.

Material Selection by Fluid

The valve body and internal parts must be compatible with the fluid being conveyed. The wrong material damages the seating surface through corrosion and the valve begins to leak. For clean water and general-purpose installations, a cast iron or ductile iron body is sufficient. In natural gas, LPG and hydrocarbon lines, a carbon steel body with stainless steel internals is used. In chemical plants and seawater applications, AISI 316 stainless steel is preferred. In steam lines operating at high temperature, the temperature class of the gasket material must also be checked.

Related Standards

The pressure class, body wall thickness and test conditions of a check valve are defined by international standards. Pressure-temperature ratings for flanged, threaded and welding-end valves are given in the ASME B16.34 standard. In pipeline applications, API 6D requirements also apply. Before ordering, the operating pressure and temperature of the line should be compared with the class rating of the selected valve.

Maintenance and Signs of Failure

As the valve contains moving parts, it requires periodic inspection. The following signs indicate that the valve should be taken in for maintenance: Draining of the line after the pump stops shows that the disc is not closing fully. Continuous knocking from the valve body indicates that the disc is oscillating. A higher than expected pressure difference between upstream and downstream of the valve suggests deposits or wear on the seating surface. External leakage from the body joint requires gasket replacement. In planned maintenance, the seating surface, disc shaft and spring are inspected; worn parts are replaced with original spares. Regular inspection preserves both the tightness of the valve and the overall safety of the line.

The Difference Between a Check Valve and a Shut-Off Valve

A check valve is often confused with a shut-off valve; however, the two perform different functions. A shut-off valve (ball valve, gate valve, butterfly valve) is controlled by the operator manually or by an actuator: it opens or closes the line. A check valve, on the other hand, is not controlled; it operates by itself. Its opening and closing are determined solely by the pressure difference in the line. This distinction matters in selection. A check valve cannot be used to take a line out of service in a controlled manner; a shut-off valve is required for this. Likewise, a shut-off valve cannot be relied upon to prevent backflow, as it depends on operator intervention. In practice, the two valves are used together: a check valve is installed first at the pump outlet, followed by a shut-off valve. In this way, backflow is prevented automatically and the line can be safely isolated during maintenance.

How Is Check Valve Size Determined?

Looking only at the pipe diameter when selecting the size is misleading. The determining factor is the flow rate in the line. A valve selected at the same size as the pipe may operate permanently half-open in a low-flow line; this means vibration and premature wear. In such cases, selecting one size smaller ensures that the valve operates in the fully open position and extends its service life. In the sizing decision, the minimum and maximum flow rates of the line are evaluated together. A selection based solely on the design flow causes problems during periods when the plant operates at partial load. In systems where the pump has variable speed, this range is wider and the selection must be made accordingly. Pressure loss curves in manufacturers’ catalogues show the minimum flow required for full opening; the selection should be verified against these curves. If it is known that the line will operate at low flow for long periods, spring-assisted models with a lower opening value can be considered.

What Are Check Valve Prices?

Check valve prices vary depending on factors such as the material used in manufacturing, size, type according to the area of use and closing mechanism. For information on check valve prices, you can contact us at export@batusan.com or by phone.

For check valve prices, you can also use our contact page.