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Pressure relief device capacity determination.

Article Excerpt
INTRODUCTION

Pressure relief devices need to have sufficient mass flow carrying capability (capacity) to limit the pressure rise in a protected component to prevent its catastrophic failure. The minimum required relief device capacity will depend on the specific component being protected and the scenarios under which overpressure is being created. Although the maximum relief device capacity is not limited by codes and standards, over-sizing relief valves should be avoided to prevent unstable relief device operation.

ASHRAE Standard 15 provides a simple method for capacity determination for both pressure vessels and positive displacement compressors (ASHRAE 2007). Because Standard 15 is not a design guide or handbook, it does not provide any additional information on other equipment where relief valves may be applied. In this paper, we present methods for relief device capacity determination intended for application on equipment that is not presently discussed in codes and standards including: oil separators, shell-and-tube heat exchangers, plate-and-frame heat exchangers, oil cooling heat exchangers, product storage tanks, and evaporative condensers. Our principle aim here is to document a basis for relief device capacity determination to insure these other types of protected components remain safe during abnormal operating excursions that can lead to high pressures. Although the methods presented in this paper are intended to apply across a wide range of refrigeration equipment and operating conditions, it is impossible to neatly prescribe relief device sizing and selection criteria to cover all situations. As such, the use of sound engineering principles and the application of engineering judgment should be always expected.

PRESSURE VESSELS

ASHRAE 15 provides a prescriptive requirement for determining the minimum capacity for pressure relief devices protecting pressure vessels. The basis for the pressure relief device capacity determination in ASHRAE is a fire condition with the heat generated from the fire radiating on a projected area of the vessel (i.e. the vessel is not fully engulfed). If other heat sources are present or contributing to the generation of vapor within the vessel, those need to be considered separately. Equation (1) shows the formula for determining the minimum required capacity for pressure relief valves protecting a pressure vessel (ASHRAE 15: [section]9.7.5).

[C.sub.r] = [Florin].D.L (1)

where

[C.sub.r] = minimum required discharge capacity of the [Florin] = relief device expressed in lbm/min of air

f = relief device capacity factor that depends on refrigerant type and whether combustible materials are in close proximity to the pressure vessel (see ASHRAE 15 for capacity factor values)

D = outside diameter of the vessel (ft)

L = length of the vessel (ft)

The coefficient, [Florin], includes an overall heat transfer coefficient attributable to radiation exchange from the fire condition (Fenton and Richards, 2003), heat of vaporization for the refrigerant (or other volatile fluid) contained within the vessel being protected, and the conversion of the refrigerant mass flow rate to an equivalent air mass flow rate. The equivalent overall heat transfer coefficient under this condition is just over 9,000 Btu/hr-[ft.sup.2] (28 kW/[m.sup.2]).

OIL SEPARATORS

ASHRAE 15 requires the application of pressure relief protection for vessels in accordance with Section VIII of the ASME Boiler and Pressure Vessel code ([section]9.7.1). One of the key provisions for pressure vessel protection is included in 9.7.2 of ASHRAE Standard 15 which states:

Pressure vessels containing liquid refrigerant and that are capable of being isolated by stop valves from other parts of a refrigerating system shall be provided with overpressure protection. Pressure-relief devices or fusible plugs shall be sized in accordance with 9.7.5.

The reference to capacity determination with [section]9.7.5 is identical to that previously presented in Equation (1). Since most oil separators, by their design and normal operation, will not contain liquid refrigerant--only liquid oil and refrigerant vapor, there is a gap in the standard to guide the process of sizing pressure relief devices for oil separators. Note that regardless of whether or not a vessel contains liquid, pressure relief protection is required by Section VIII Division 1 of the ASME Boiler and Pressure Vessel (B&PV) Code.

To assess the pressure relief capacity requirements of an oil separator, a transient analysis of the mass and energy flows associated with a pressure relief process for an oil separator was conducted. The analysis compares relief capacities required to prevent overpressuring the oil separator with relief device sizing criteria previously given in Equation (1) for three alternative scenarios. Details of the governing equations used in the analysis are given by Reindl and Jekel (2006). Table 1 summarizes the cases analyzed and the initial conditions present just prior to the relief valve protecting the oil separator lifts.

Table 1. Summary of Conditions for Transient Analysis of Over-Pressurizing Oil Separators. Case Description Initial Conditions 1 Booster compressor oil 165 psig (11.4 bar) with a separator, idle and isolated coincident gas temperature of prior to external heat 800[degrees]F (427[degrees]C) due addition to external heat addition 2 High stage compressor oil 275 psig...

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