Case 2 Failed Check Valves

Two PFA-lined ball valves failed after six years in service, which is considered premature failure. In the most recent period, the orientation of both valves had been changed from horizontal to vertical. The process stream operated at a temperature of 90°C and contained a mixture of acids including nitric, sulfuric, oxalic, picric, and styphinic at a pH of 1.3. The failure manifestation was unexpected and there was excessive audible pinging during the process operation. This case is an example of a failure where analysis has to be conducted by knowledge of the material, equipment, and the process, in contrast to the collection of definitive data. The ball had a diameter of 17 cm and weighed over 

The valve had a PFA liner, which was the appropriate material for this application from a chemical resistance standpoint. A close-up picture of the interior surface ofthe valve is seen in Fig. 10.37. One of 

Polymer Treatment Surface Chemical Analysis (%) by ESCA  

Close inspection of the valve prior to installation and measurement of the dimensions at the installation of the new valve and during periodic inspection were the recommended corrective actions. 

A glass-cloth coated with FEP exhibited contamination in the form of black spots resembling carbon black. The coating process did not use any carbon black, thus ruling it out as a source of contamination. What was the nature of contamination and its possible sources  

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Check Valves. Check valves are used in gas lines where it is important to assure that gas can only flow in one direction. They are not recommended for use as the primary means of shutoff but as a backup should the primary shutoff valve fail. They are commonly used in systems on the purge gas inlet line to assure that process gas will not back up into the purge gas supply. This is especially important if a common purge gas reservoir supplies many gas delivery systems. In some systems, many different gases may be vented into a common header. If this is the case, separate check valves should be installed on the vent lines for the separate gases to assure they will not be able to mix in the system. Check valves can also be used on the process gas delivery line to prevent any backup of gas from the process into the gas supply system. They can also be placed directly after the cylinder to prevent any backup and contamination of the cylinder. 

In some cases, after careful evaluation, other valves may be considered for EBVs such as spring-loaded control valves that fail closed, back flow check valves (these are not normally considered reliable enough for EBVs by many engineers), and excess flow valves. Excess flow is the loss of material from the confined environment of a vessel or pipeline. Two approaches are available for the detection and valve action of excess flow valve systems (1) External, where excess flow is detected outside the valve itself, and (2) internal, which is within the valve unit and has limited applications. Excess flow conditions are detected more readily because of loss of resistance to flow than because of loss of pressure. All excess flow detection systems are based on product physical properties as well as flow rate. A change of products or process conditions may require a change in the excess flow detection system. For example, a number of excess flow valves are rated for low pressure and are made of materials not suitable for hydrocarbons. 

A disadvantage to the use of alkanes is their very low viscosity, which leads to potential instrumentation problems. The low backpressure generated in the system often fails to seat outlet check valves, leading to erratic flow rate characteristics and, consequently, variable retention times. Therefore, a flow restrictor may be needed in these cases to produce an artificial backpressure on the check valve. The restrictor is... 

Interfacing nuiintenance demands When maintenance is completed on equipment, for example, a pump, a complete functional check of it and its associated equipment (e g., isolation valves), is usually performed before the equipment is returned to service. It is important to note that in this case the interface demand is placed on the associated equipment, not the equipment that failed. Interfacing maintenance demands are counted separately from failure-related demands. 

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