Causes of Overpressure – Hydraulic Expansion

The Hydraulic (or Thermal) Expansion causes of overpressure is applicable to an equipment item whenever all of the following three conditions are met:

a. The item can be blocked-in
b. The item is full of liquid.
c. A mechanism exists to heat the blocked-in liquid. The source of heat may be any of the following:
· Process heat (e.g., from a fired heater or heat exchanger)
· Heat tracing by steam, hot water, electricity, or other means.
· Solar Radiation
· Ambient conditions (when the “cold” liquid is at or below ambient temperature).

When all three of these conditions prevail, the temperature increase leads to thermal expansion of the liquid. Because the equipment is liquid-full, this expansion produces a dramatic increase in the equipment’s internal pressure, unless a relief path is available to the expanding liquid.

Equipment types to which this contingency typically applies are:
a. Pressure vessels that operate liquid-full and at or below ambient temperature (e. g., filters and adsorbers).
b. Heat exchangers (including heat-traced piping).
c. Pipe segments that operate liquid-full and at or below ambient temperature.
d. Some block valves that trap small amounts of liquid when in the fullyopened or fully-closed position.

The cold sides of heat exchangers always have a source of heat available to produce a temperature increase. If the cold side should become blocked-in liquid-full while flow continued on the hot side, the Hydraulic Expansion contingency would require pressure relief. Similarly, liquids normally at or below ambient temperature have a potential heat source in both the ambient air and in solar radiation. Note that this can even apply to the hot side of a heat exchanger if both sides operate below ambient temperature.

At some facilities, operation and maintenance procedures and training are established to ensure that the hot side of a heat exchanger is blocked-in before the cold side, or that the cold side is drained before being blocked-in. In this way, the cost of purchase, installation, maintenance, and documentation of a relief device can sometimes be avoided. At other locations, reliance on such procedures is not consistent with the local safety-management philosophy. It is important to note that, when documenting the pressure-relief contingencies for the exchanger in question, Hydraulic Expansion should still be identified as an applicable contingency, even if the above-mentioned practices are used. In the documentation, these practices should be noted as in use, removing the need for a relief device.

The required relief flow rate for any Hydraulic Expansion contingency can be calculated from the trapped liquid’s heat capacity and thermal expansion coefficient, together with the duty of the heat source. The first two variables are straightforward properties of the liquid involved. For some types of heat sources, the duty is usually established with little difficulty. This is the case for the cold sides of heat exchangers, for heat-traced piping, and for any equipment for which solar radiation provides the heat source (it is conservative to assume a solar heat flux of 350 Btu/hr/ft2). However, when the atmosphere is the source of heat to a sub-ambient trapped liquid, the heating duty is not easily established. Fortunately, this duty is small enough that its determination is typically not required. API Recommended Practice 521 points out that it is common practice to use a ¾” x 1″ NPS relief valve in most installations requiring thermal expansion relief. Experience has shown that this size device typically provides adequate relief capacity. Possible exceptions noted by Recommended Practice 521 include long, uninsulated pipelines and large vessels. In these cases, it is the usual practice to use the solar flux of 350 Btu/hr/ft2 and the exposed surface area of the equipment item to estimate the heat duty to the item.

Finally, because the relief flow rate required for this contingency is often a nominal value, the fact that most block valves leak becomes relevant. In some cases, the leak rate through some “closed” valves may be large enough that a thermal relief valve becomes unnecessary. Leak-through flow rates are available from many valve manufacturers, so a quantitative analysis of the situation may be possible.

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