Causes of Overpressure – Failure of Automatic Controls

Most control valves are designed either to go to a fully open or fully closed state, or to retain their position upon the loss of their actuating signal or driving medium (often instrument air). Regardless of this “failure position” design, a failure of the primary control element can always cause the controlled valve to go fully open or fully closed. Such a control failure that causes a valve at an equipment outlet to close should be evaluated as either a potential Closed Outlet or Overfilling contingency, as described above. A control failure that causes an inlet valve to open can create a cause of overpressure for downstream equipment if the pressure upstream of the control valve can exceed the equipment’s design pressure.

The two criteria that must be met for inlet control valve failure to qualify as a potential causes of overpressure are the following. In evaluating an equipment item for the applicability of control failure, it is necessary that all inlets be identified and checked against these criteria.

a. A control valve or regulator must be present on an inlet line to the equipment; and
b. The normal operating pressure upstream of the control valve or regulator must be greater than the design pressure of the equipment being evaluated. In the event that the normal upstream operating pressure is unknown, 90% of the upstream relief device set pressure may be used.

A special case arises when the downstream side of the control valve connects to a header system with parallel outlets. In these cases, failure open of the control valve may not result in overpressure of equipment since the upstream pressure can be relieved through several outlets. Another case of special concern is that involving failure of a level control valve (LCV) that dumps high pressure liquids into a low pressure system. While the increase flow of liquid will place an increased burden of vapors flashing from the liquids as they are let down into the low pressure equipment, the more significant causes of overpressure occurs after all liquid has drained from the upstream vessel. The high pressure gases that then flow through the failed LCV can rapidly create serious overpressure in the downstream equipment.

In general, the required relief flow rate for an inlet control valve contingency is evaluated as the increase in the flow into the equipment relative to the normal operating flow rate. The full-open flow rate should be evaluated with the downstream side of the control valve at relief conditions. The relief device should be sized using full size trim in the control valve, even if the actual control valve has reduced trim. It should be noted that these conditions might involve a change in the fluid phase from normal operating conditions. Credit may be taken for the increase in flow through the equipment’s outlets due to the increase in the equipment’s pressure. API Recommended Practice 521 recommends that in taking credit for this increased flow, any valves on the equipment’s outlets are assumed to be in their normal operating positions.

In all cases, consideration should be given to the possibility that the control failure under consideration may affect more than one control valve. In some cases, the failure could cause multiple inlets to a vessel to go open. On the other hand, an inlet may go open while an outlet goes closed. In such a case, of course, it would be inappropriate to take credit for the normal flow through the closed outlet.

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