IPE-TM-300 Vessels
Vacuum Design
IPE-TM-300-05
1. Purpose
This procedure states Inflection Point Engineering’s practices for designing equipment for vacuum conditions (external pressure).
2. General
The ASME Code, Section VIII, states that vessels operating under vacuum must be designed for an external pressure condition. As such, design any equipment that requires evacuation, or may undergo vacuum during some phase of operation for an external pressure condition.
The shells of vessels that are designed for a vacuum condition often require stiffening rings. As indicated in Attachment 1, Figure 1, tray support rings afford very little stiffening. Also, Attachment 1, Figure 2 indicates that fractionators designed for 50 psig internal pressure may need to have stiffening rings installed in order to withstand Full Vacuum.
3. Guidelines
Apply the following guidelines to each specific situation.
- Design all equipment for Full Vacuum if the equipment normally operates under a vacuum condition, or the equipment is subject to start-up, shutdown, and/or regeneration evacuation.
- Design all vessels and heat exchangers for Full Vacuum if they normally operate liquid full and can be blocked in, cooled down (from normal operating to winterizing temperature), and be subject to vacuum conditions. This requirement shall not apply to the cooling side of heat exchangers in cooling water or seawater services. This requirement also does not apply to LPG service where vacuum conditions typically do not exist at the winterizing temperature.
- Design all fractionators and associated equipment for Full Vacuum if the loss of heat input and cool down can cause a vacuum condition even if the fractionator/receiver floats on the relief header. Associated equipment includes condenser, receiver, reboiler, feed-bottoms exchanger, and possibly a bottoms cooler. Consideration should be given to the static head of liquid which could exist during a heat loss situation.
- Design all vessels and exchangers in steam service, which may be blocked in and cooled down for Full Vacuum. The Full Vacuum design temperature shall be specified as 250°F (120°C).
- The winterizing temperature (see section 4.1 of the BEDQ) should be used to determine if full vacuum design conditions should be specified due to loss of heat input. This can be accomplished by determining the bubble point pressure using a thermodynamic properties simulator upon the overhead liquid stream at the low ambient temperature. If the resultant pressure is less than 14.7 psia, a full vacuum design is required.
- Address special start-up/shutdown/regeneration procedures by consulting with the appropriate Technology Center Specialists and establishing the design temperature for reactor sections associated with Full Vacuum.
- For other services, set the vacuum design temperature of equipment that is designed for Full Vacuum equal to the normal operating temperature of the equipment. For heat exchangers, use the highest operating temperature as the design temperature for Full Vacuum.
- Blocking in after steam out, is considered operator mis-operation and is not, in general, used by Inflection Point Engineering as a case for designing for an external pressure condition. If the Customer requests equipment be designed for this mis-operation (see section 2.3 of the ), indicate design external pressure as Full Vacuum and vacuum design temperature as 250F (120C) unless Customer indicates higher temperatures in the BEDQ.
- Use the term “Full Vacuum” to specify external design pressure. Do not express the external design pressure numerically unless customer requests otherwise.
- If the customer requests the vacuum condition to be expressed numerically, then it shall be done according to the following formula: where PAtmospheric = 15 psia (0 psig). PIntermal must be expressed in the same units as the external pressure; e.g. full vacuum =0 psia (-15 psig). Do not mix gauge and absolute units.
- Note that the atmospheric pressure (PAtmospheric) is always taken as 15 psia (1.05 kg/cm2a), per the ASME code Section VIII – Division 1 UG-28(f). Do not use 14.696 psia (1.0332 kg/cm2a) or 14.7 psia (1.0335 kg/cm2a) as the atmospheric pressure.
- The external pressure is always expressed in units of differential pressure (i.e. psi, bar, kPa or kg/cm2). Do not express the external pressure as gauge or absolute pressure.
- Example: The customer requests an external pressure of “half vacuum” to be expressed numerically. The internal pressure = half vacuum = 0.5 * 15 psia = 7.5 psia. = 15 psia – 7.5 psia = 7.5 psi. Indicate an external design pressure of 7.5 psi on the vessel specification.
- Consult with the Customer during the design basis discussions to identify any specific requirements for external pressure design.
Attachment 1
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