Controls & Instrumentation Product Manual - Spirax Sarco

4
How To Use This Book
Accepted international designations for limiting conditions are used throughout this catalog.
An explanation of Pressure Shell Design Conditions and Limiting Operating Conditions follows:
Pressure Shell Design Conditions:
• PMA - Maximum Allowable Pressure
• TMA - Maximum Allowable Temperature
These are the maximum pressures and temperatures to which the pressure shell (body, cover, bolting,
etc.) of the product may be safely and permanently exposed. The product may or may not operate
properly at these maximum conditions; the internal parts may be damaged or destroyed, but the pressure-retaining
components will not break or become permanently distorted. It is sometimes permissible to
exceed the maximum allowable conditions temporarily. For example, a cold hydraulic test at 1-1/2 times
the cold PMA is usually permitted.
Because the maximum allowable pressure depends on the temperature, there is no one value for
PMA. Quoting only the cold PMA without mentioning the corresponding temperature could be misleading.
PMA is usually given at three temperatures:
• Cold
• Maximum Allowable Saturated Steam Temperature
• Maximum Allowable Temperature
There is only one value for maximum allowable temperature (TMA); the corresponding pressure
range is given in this literature.
PMA and TMA are strictly safety related. They have very little to do with the actual operation of the
product.
Limiting Operating Conditions:
• Maximum Operating Pressure - PMO
• Maximum Operating Temperature
PMO is the maximum fluid (steam, gas or liquid) pressure at which the product will operate properly.
Depending on the type of product, PMO may or may not be related to temperature. Unless otherwise
indicated, the downstream pressure is assumed to be O psig.
Maximum operating temperature may depend on superheat tolerance, internal materials (e.g. neoprene
o-rings), pressure shell design conditions, or any combination of these factors. Depending on the
type of product, it usually varies with the actual operating pressure.
An example on the opposite page graphically illustrates limiting conditions.