Valve chattering

Inlet piping is held to a minimum, with the safety device preferably mounted directly on the equipment and with the total system pressure drop loss to pres.sure relief valve inlet not exceeding 3% of the set pressure in psig, of maximum relief flowing conditions [10]. To conform to code (see ASME code. Sect. Vlll, Div. l-UG-127 [1]) avoid high inlet pressure drop and possible valve chatter ... 

In the mean time look at the change which has taken place in the water to which we have applied heat—it is losing its fluid state. You may tell this by two or three circumstances. I have covered the mouth of this glass flask, in which water is boiling, with a watch-glass. Do you see what happens It rattles away like a valve chattering, because the steam... 

Valves should be installed vertically as shown in Fig. 3. Care must be taken that the internal diameter of the pipe fixing is equal to or greater than the valve bore and the connector is short enough to limit pressure loss between the reservoir and bore to 3% of set pressure. If the pipe-fixing diameter is too small then the valve may not discharge at full capacity. Moreover, instability may be induced because of excessive pressure drop in the pipe connector. Rapid and repetitive rise and fall of a valve disk is referred to as valve chatter. It can also be induced by misapplication of a valve intended for gas service on liquid service. 

Frommann, O. Friedel, L. Analysis of safety valve chatter induced by pressure waves in gas flow. J Loss Prev. Process Ind. 1998, II (4), 279-290. 

The practice of using multiple relief devices at staggered settings is one of the most desirable applications of this technique (10). This practice reduces relief valve size, thus reducing leakage, valve chatter, and seat damage. It also lowers the reactive thrust at relief. Further discussion is available elsewhere (10). 

Relief valve chattering. Relief valve set point too close to operating point of pump. Reset relief valve or replace with another valve with higher set point. 

A relief valve must be removed from service for inspection and repair after an online test if it fails to open at set pressure or the valve chatters, or if it fails to reseat properly. 

For service required to handle several different relief cases and required difTerent relief sizes, it may be better to use two or more relief valves instead of a big relief valve. This may avoid relief valve chattering at small relief case. 

Rapid eycling can occur when the pressure at the valve inlet decreases at the start of relief valve flow beeause of excessive pressure loss in the piping to the valve. Under these conditions, the valve will cycle at a rapid rate which is referred to as chattering. The valve responds to the pressure at its inlet. If the pressure decreases during flow... 

An oversized relief valve may also chatter since the valve may quickly relieve enough contained fluid to allow the vessel pressure to momentarily fall back to below set pressure only to rapidly increase again. Rapid cycling reduces capacity and is destructive to the valve seat in addition to subjecting all the moving parts in the valve to excessive wear. E.xcessive back pressure can also cause rapid cycling as discussed above. 

External relief valves should be of the full-flow, non-chattering (modulating) style. In oil systems with relatively low pressure levels, the hydraulic-type relief valve becomes attractive because the plug lift is smooth, and instability during lifting is eliminated. The valves should be located as close to the pump as possible to provide fast reaction time. 

Once the set pressure is reached, the valve opens fully and remains open, so long as the set point is exceeded. There is no need for overpressure or minimum flow to keep it open. Thus, it is not subject to chattering at low discharge rates. 

When the pilot exhausts to the atmosphere, a pilot-operated PR valve is fully balanced. Like the balanced bellows valve, therefore, its opening pressure is unaffected by back pressure, and high built-up back pressure does not result in chattering. 

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

Balanced bellows PR valves need not be restricted to the same built-up back pressure limit (10% of set pressure) as are conventional valves, since they are not subject to chattering from this cause. However, maximum back pressure is limited by capacity and in some cases by the mechanical design strength limitations of parts such as the outlet flange, bellows, or valve bonnet. 

In addition, a further mechanism of chattering may be introduced in some liquid PR valve installations if the response characteristics of a control valve in the same system are such that hunting between the two occurs. Generally, this can be eliminated by adjustment of instrument settings or by installation of two valves with staggered set points... 

Liquid service PR valves are characterized by progressively increased lift with increasing inlet pressure, rather than the "pop" action of vapor service valves. Liquid service valves are, therefore, less liable to chattering at low relieving rates, and they will modulate down to about 10% of design flow. 

Excessive Built-up Back Pressure - Built-up back pressure resulting from discharge flow through the outlet system of a conventional PR valve results in a force on the valve disc tending to return it to the closed position. If this returning force is sufficiently large, it may cause the valve to close, only to reopen immediately when the effect of built-up back pressure is removed. Chattering results from the rapid repetition of this cycle. 

Where outlet pressure losses exceed 10%, bellows valves are often considered. However, substitution of a bellows valve for a conventional valve may not necessarily solve the chatter problem since debits associated with bellows valves reduce the rated capacity of this type valve. Hence, the valve has a tendency to become oversized depending on the amount of back pressure encountered. For this reason, revision of outlet piping to reduce the back pressure within the 10% limit is strongly preferred to the alternative of installing a bellows valve. 

Number of Valves Required - Normally a manufacturer s standard PR valve with Drifdce area equal to or larger than the calculated requirement is specified. In some cases, e.g., large reheving rates or to prevent chattering, two or more valves are necessary. Likewise, if there is an appreciable difference between the calculated orifice size and the available size, multiple PR valves are desirable to more nearly match the available area to the required orifice area. The column for "spares" indicates the requirement, if any, for spare PR valves installed on the equipment. Normally, this applies only in the case of refinery preference or local regulations, but is required in many European countries. 

In the case of set pressures below 100 KPa gage (since pressure drop becomes proportionately larger) the maximum inlet pressure drop at design flow rate may be extended to 5% of set pressure (KPa, gage). The purpose of this requirement, which applies to both conventional and bellows type valves, is to prevent chattering. 

Chattering caused by undersized inlet piping may sometimes be eliminated on pilot operated PR valves if the pilot valve pressure tapping is taken directly from the vessel being protected. However, it is recommended that the above inlet pressure drop limitations still be applied, to avoid the capacity reduction that would result from excessive inlet losses and to ensure freedom from chatter. 

Chatter The abnormal rapid, reeiproeating motion of the movable parts of a pressure relief valve in whieh the disk eontaets the seat. 

---