In this this article, guest author Mahadevan, V discusses the types of pressure vessels according to their designs. It also covers the minimum considerations needed for safe operation of the pressure vessels during the design stage and during plant operations. He also mentions the types of process vessels, heat exchangers and storage tanks.
What is a Pressure Vessel?
A metallic / nonmetallic container, generally cylindrical or spheroid, capable of withstanding various loadings and in which materials are processed, treated, or stored. (We refer to metallic containers in this write-up.)
The most common types referred as static equipment in the EPC (Engineering, Procurement, Construction) industry of Refinery / Petrochemical / Chemical Process plants are:
Design Methodology for Pressure Vessels
Pressure vessels are normally designed for the following:
◦ DP – Design Pressure is the pressure at which the contents are processed in the vessel.
◦ DT – Design Temperature is the temperature at which the above pressure is maintained.
◦ CA – Corrosion Allowance – based on corrosion rate of product stored inside the vessel & the expected life of the vessel.
◦ Environmental loads – Wind / Seismic loads
◦ Attachment loads – The loads induced by pressure safety valves, deluge system, connected piping, flow control instruments etc
The above loads are translated into stress and compared against an allowable stress.
◦ Allowable tensile stress – minimum of (Ultimate tensile stress/FS, Yield stress/FS)
◦ Allowable compressive stress – steps given in design code
◦ FS – Factor of safety – the values defined in the vessel design code or decided by the design engineer
(The reason why mill material certificates are required along with delivery of plates)
The Typical / Common Pressure Vessel Design Codes
***kPa – Kilo Pascal
Minimum Safety Considerations for Pressure Vessels
The minimum considerations for safe operation of the vessel are:
At Design Stage
▪ Design pressure (DP) – The design pressure is established based on the most severe operating pressure * pre-defined safety multiplying factor
▪ DT –The design temperature is established based on the most severe operating temperature + pre-defined safety addition factor
▪ NDT / NDE – The extent of non destructive test (i.e. radiography, Ultrasonic test, Liquid penetrant test etc) determine the factor which further reduces the allowable stress
▪ All pressure vessels after fabrication are subject to hydro test (Pressurized water filled test) for a short time (approx. 30 – 60 minutes) to detect leaks / flaws in the weld joints & bolted connections of the vessel.
▪ MDR – A Manufacturer’s Data Record is submitted by the vessel manufacturer to the purchaser, which has records of material traceability, weld methods utilized in fabricating the vessel, NDT performed, record of hydro test, etc. apart from the design calculation and drawing of the fabricated vessel.
Vessel in Plant Operation
▪ Fireproofing – If a vessel is located in hazardous / flammable area of the plant their support, which is connected to the foundation, are passive fire protected to retard the effect of structural damage (approx. 30 minutes min to 2 hours) in the event a fire related accident occurs.
▪ Over pressure protection – During the plant operation, if an upset pressure arises in the vessel, pressure safety valves (PSV) are installed at the highest point of the vessel to relieve the pressure.
▪ Control instruments – Vessels are mounted with level instruments, temperature indicators, pressure indicators to monitor the parameters like liquid level, temperature and pressure inside the vessel and relevant signals are sent to the control room for controlling the measured parameters.
▪ Periodic inspection of pressure vessel are conducted like visual inspection of the complete vessel, monitoring the thickness by ultrasonic method (to estimate rate of corrosion) etc. Any alteration or observation in the vessel is to be recorded.
▪ Re-hydro test of vessels are performed at an interval specified by local statutory body.
Types of Process Vessels
Process Vessels are those which specific process is being done. Examples of process vessels are:
They are vertical vessels filled with mass transfer device like Trays or Packing’s. The purpose of the mass transfer device is to increase the contact area for effective mass transfer / separation.
As the name implies, they are to facilitate desired reaction of liquid / vapor which are typically equipment with special internals / catalysts / agitators etc.
Separators are those pressure vessels where 2 phases of mixed streams are separated. For example, a fluid containing vapor which needs to be separated is passed into the sized separator where the vapor phase is separated and the liquid phase is drained from the bottom of the vessel.
They are typical vessels which serves the purpose of storing the following:
– Unfinished petroleum / chemical components awaiting for further processing (like buffer vessel / surge vessel)
– Collect process liquid before the next process (like closed Blow Down drum)
– Knock out drum – To knock out liquid component and let the vapour component out
Types of Heat Exchangers
Heat exchangers are those equipment where heat energy is exchanged for energy optimization, product storage or specific process requirement. They are exchanged either directly (eg. Cooling Tower) or indirectly (eg. Shell and Tube Heat exchanger).
Types of Storage Tanks
Storage Tanks are to store the following:
- Utilities like fuel oil, fire water etc
- Finished products prior to shipping out to customers
- Unfinished petroleum / chemical components awaiting for further processing and / or blending
The common types are:
Below are a few components of Metallic Pressure Vessels where pressure vessel steel plates are used:
To conclude, pressure vessels are tailor-made equipment which are sized, designed individually to meet the service / operating need of a process plant.
Each fabricated vessel for a process plant is unique and has a fabrication / operating history until it is decommissioned from service.
Hence, even to interchange identically dimensioned vessels for a different service (other than its intended service) needs a re-visit of both MDR & plant operations record.