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A check valve (Figure A5-3), prevents flow reversal, thus preventing damage caused by the reverse flow velocity at which the fluid reaches a pump station, or an upstream location along a pipeline route at a lower elevation, where a pipeline segment could be vulnerable to surge. It effectively reduces the pressure surge. However, check valves may end up being located in remote locations where road access can be challenging. There are several types of check valves available including:

Swing clapper (top swing) ·

Swing ball ·

Spring assisted split disk /wafer double disk ·

Nozzle/annulus type ·

The pipeline industry mostly uses the swing clapper type as these are through bore type of valves, thus facilitating the passage of pipeline pigs.

A check valve has a clapper type moving part (Figure A5-3, inset). During normal flow, the clapper swings up (in the direction of the flow, Figure A5-4), allowing free forward flow. When the pressure is lost or when the downstream pressure is higher than the upstream pressure, the clapper closes. The clapper falls and creates a barrier to back flow usually in milliseconds (Figure A5-4, inset B). The higher the downstream/ backflow pressure, the tighter will be the seal. It may be noted that some clapper types (on side hinged check valves) are known to vibrate and sometimes stick.

Advantages of using a check valve are:

Effective in preventing the surge pressure damaging vulnerable pipeline ·

Pipeline Operation and Batching n 283

Minimal maintenanceProtecting pipeline and facilities from highest peak ·

pressure

However, it may be noted that check valves are not considered an adequate form of isolation and they do require maintenance. Hence, check valves should be installed in conjunction with upstream and downstream isolation valves as recommended by the industry.

Check valve seats are subject to cavitation/erosion where elevation changes in a pipeline result in liquid column separation due to pressures falling below the vapor pressure of the liquid. In these situations, non-slam check valves are preferred.

Figure A5-4. Check valve operation (A — clapper open, B — clapper closed) [20]

Figure A5-3. Typical pipeline check valve [20] — top inset air cushioned swing check

The ideal check valve is one that closes immediately when the flow velocity at the valve reaches zero and/or when downstream pressure is higher than upstream pressure. This velocity reduction to zero is likely to control valve slam but not necessarily maintain pressures at acceptable levels. Potential for a high pressure surge downstream of the valve must be analyzed and the adjoining piping checked for suitability. The decision as to the best type of valve in a particular installation depends on the characteristics of the valve and the pipeline system (Figure A5-5).

Valve dynamic characteristics should be checked against pressure transients in pipeline system design. The deceleration flow rate is the most important parameter and it can be determined by first analyzing the system without the check valve. When the fluid deceleration rate has been determined, then the maximum backflow/reverse velocity Vr can be determined from the dynamic characteristic of the check valve and thus the suitability of the check valve for the application [18].

From Figure A5-5 above, it is evident that a nozzle/annulus type check valve with strong springs allows the lowest maximum reverse velocity to develop and hence this type of check valve (Figure A5-6) is closest to the ideal check. This particular non-slam check valve has a rapid closing time of less than 0.4 s. When flow reverses, the valve is already closed and thus the pump can be protected from any reverse high pressures [2].

Figure A5-5. Comparison of dynamic characteristics of check valves (reproduced from [18])

Pipeline Operation and Batching n 285

Many facilities exposed to check valve slam with conventional swing check valves have been made silent by the use of the nozzle type check valve. There are two types of nozzle check valves; single spindle single spring or annulus multiple spring type.

As an example, a plot of dynamic pressure/head transients downstream of such a check valve (located downstream of a pump that has been tripped) is shown in Figure A5-7. As the check valve is able to respond quickly to changing flow condition, the maximum back flow velocity Vr is small. However, if the valve motion was slow to respond, the associated pressure rise will be much larger due to a much higher back flow velocity Vr.

However, while this type of check valve is suitable for pump station protection, its use is inappropriate in transmission pipeline application where a full bore application is required for pigging. Mainline swing check valves can be fitted with a device called a slam retarder. This is a device designed to prevent the clapper of a check valve from slamming as it closes upon flow reversal.

Note on check valves for mainlines: the US DOT Office of Pipeline Safety (OPS) now known as the Pipelines and Hazardous Materials Safety Administration (PHMSA), alerted the operators of hazardous liquid pipelines to test check valves located in criti- cal areas to assure the proper closure during a pipeline failure. The failure of such valves to close during an incident could increase the risk to public safety or damage to the environment.

This alert was due to a pipeline incident that has caused PHMSA to reevaluate the safety of pipeline check valves. The clapper in these valves had hinges on the side rather than at the top. As a consequence a top hinged clapper is preferred along with a surge analy- sis to simulate the functionality of check valves in pipeline transient flow situations.