Relief Valve

Table of Contents

  • Introduction
  • Parameters
  • Sizing
  • Specifications

Introduction

The relief is used to vent content when pressures exceed safety limits.

Parameters

The set pressure is the inlet pressure at which the valve starts to open. If the pressure reaches or exceeds the full open pressure, then the valve will be fully open, allowing material to vent.

Sizing

The following valve control types are available:

  • Quick opening: Large increase in flow with a small change in valve stroke.
  • Linear: Valve travel is directly proportional to the valve stoke.
  • Equal Percentage: Equal increments of valve travel produce an equal percentage in flow change.

The flow rate is also determined by the flow area, and the various discharge coefficients, and are approximately as follows:

Flow = (Vapour fraction) * (Vapour flow) + (liquid fraction) * (Liquid flow)
Liquid flow = Area * LK1 * sqrt (Density * Pressure gradient)
Choked vapour flow = Area * VK1 * sqrt (Density * Inlet pressure)
Non-choked vapour flow = Area * NCVK1 * sqrt (Density * (NCFLow1 - NCFlow2))
NCFlow1 = P ^ ((CpCv-2)/CpCv) * (Outlet Pressure) ^ (2/CpCv)
NCFlow2 = P ^ (-1/CpCv) * (Outlet Pressure) ^ (CpCv+1)/CpCv)

LK1, VK1 and KCVK1 are typically equal to the valve opening multiplied by applicable discharge coefficients.

The flow is choked if the pressure ratio exceeds the choking constant which is (2/(CpCv+1)) ^ CpCv/(CpCv-1).

Specifications

Choose whether the valve opening is directly determined by the pressure only (i.e. the opening will always be the same at a given pressure regardless of whether the valve is opening or closing), or whether it exhibits a more complex hysterysis. In the latter case, the valve opening is also determined by whether the valve is busy opening or closing, and not just by the pressure. This option is commonly used. If the pressure starts to decrease with the valve open, it will start to close at the closing pressure and be fully closed by the time the pressure has dropped to the reseating pressure.

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