Compressed Air Pressure Vessel

Introduction

Pressure vessels are used for numerous applications and are split into two main groups, un-fired and fired. Un-fired pressure vessels are those which are not exposed to any form of combustion, such vessels include dive cylinders and gas storage cylinders. Fired pressure vessels are those which are exposed to some form of combustion, such vessels include fire tube and water tube boilers.

The saVRee 3D model is an unfired pressure vessel designed for use with a compressed air system. Throughout this article we will assume that we are discussing a pressure vessel used in a 6-8 bar(g) compressed air system.

Why do we need pressure vessels?

Pressure vessels represent stored energy in a similar way as a battery represents stored energy. There are several reasons for storing energy within pressure vessels.

System Pressure Fluctuations

Demands upon the system may vary and pressure vessels allow us to meet these demands without causing any service interruption. For example, if many people in a factory were to use pneumatic tools at the same time, the demand on the compressed air system would increase quickly. If no pressure vessel was installed, the pressure within the compressed air system will drop quickly and potentially cause service disruptions in other areas.

Heat Exchanger

Discharged compressed air from a compressor is hot. By storing the compressed air within a pressure vessel we are transferring some of that heat to the surrounding air. A reduction in heat is accompanied by a reduction in pressure which raises the overall system efficiency.

Damper

Compressors starting and stopping cause pressure fluctuations/pulsations, but a pressure vessel dampens these and delivers steady constant pressure which is better suited for end consumers.

Loading and Un-Loading of Compressors

Any pressure fluctuations within the compressed air system will need to be dealt with by the compressors. If the pressure is too low, a compressor will start and increase the system pressure until such time as it receives a signal to stop. A small system without a pressure vessel would require the compressor to start and stop often; this is known as short-cycling and is not beneficial to the compressor.

Moisture Removal

Although most modern systems employ driers to remove moisture after compression, some moisture carryover may still occur. The cooling that occurs within the pressure vessel aids the forming of condensation which can be easily removed from the system using the condensate drain.

Construction

The pressure vessel represented by the 3D model is relatively standard; the shell is cylindrical in shape whilst the ends (‘dishes’) are rounded. A round shape with no sharp corners ensures that the pressure is distributed as evenly as possible. Any corners or sharp edges used in construction are referred to as ‘stress raisers’ and should be avoided if possible as they weaken the vessel’s structural strength. If the vessel is not manufactured from one single sheet of metal, it is normally welded, although older vessels were often riveted.

The vessel itself is usually constructed of steel. The exterior of the vessel is covered in primer or paint whilst the interior is often bare steel. Some vessels have interiors that are epoxy or resin coated to increase their corrosion resistance. Vessels constructed of stainless steel are unusual, but are sometimes used within industries that require higher air purity e.g. medical and semiconductor industries.

Appendages

Several pipes of varying sizes are connected to the pressure vessel. The large two pipes shown on this vessel are for the compressed air entrance and discharge. The smaller pipes are for the fixing of the various appendages needed to operate the vessel efficiently and safely. Some common vessel appendages are listed below.

Local Pressure Gauge

A pressure gauge mounted directly to the vessel gives personnel a local visual indication of the vessel pressure. Gauges can usually be isolated from the tank using a valve; this makes replacement of the gauge possible without the need to drain the pressure from the whole vessel. The bourdon type gauge is the most common pressure gauge employed today. Gauges fitted to systems with very high pressures should be equipped with shatter-proof glass as standard.

Bourdon Pressure Gauge

Bourdon Pressure Gauge

Differential Pressure Switches

These types of switches control the start (cut-in) and stop (cut-out) signals to the compressor; the switches are commonly referred to as ‘delta P’ switches. Delta P switches are also used for the setting of alarm points at specified pressures e.g. high vessel pressure will actuate the delta P switch and activate a visual and audible alarm.

Safety Relief Valve (SRV)

Spring loaded safety relief valves (SRVs) are now standard on almost all pressure vessels above a specified size and pressure. It is good practice to mount the SRV directly to the vessel without any means of isolation (valve) between the SRV and vessel.

Safety Relief Valve

Safety Relief Valve

In the past, counterweight SRVs and spring loaded SRVs could be used, but counterweight SRVs have now fallen out of favour as they are less reliable than the spring loaded type.

The SRV spring tension is calculated so that it keeps the valve closed unless a specified pressure within the vessel is reached, at which point the spring will compress and the valve will open.

Condensate Drain Valve

The lowest point within the vessel is fitted with a drain to remove condensate. It is important to remove condensate to prevent corrosion of the vessel’s interior. Driers remove moisture from the compressed air prior to it entering the vessel, but condensate drains should still be installed and operated to ensure the drier is working correctly. There are two main methods for draining condensate from the vessel.

The first is the traditional manually operated valve that must be opened at scheduled intervals in order to drain the condensate. This solution is the simplest but relies on personnel to actually perform the task; this can create a situation where the drain is not opened as often as it should be.

The second is an automatic condensate drain that discharges the condensate at scheduled intervals for a set period of time e.g. once a day for 30 seconds. Theoretically the automatic condensate drain is the better option although a failure of the automatic drain may go un-noticed by personnel and condensate will accumulate until such time as the failure is noticed.

Fusible Plug

A SRV protects the vessel from over pressure whilst a fusible plug protects the vessel from over temperature. A fire in the immediate vicinity of the vessel would increase the temperature and pressure within the vessel. In order to avoid an explosion, the fusible alloy within the fusible plug will melt and the pressure will be released.

Fusible Plug

Fusible Plug

Inspection Ports

Inspection ports may be sight holes, hand holes or manholes. Inspection ports allow personnel to view the interior of the vessel and perform a condition assessment. It is also possible to access the interior of the vessel through the inspection port for cleaning purposes.

Maintenance and Testing

Requirements concerning maintenance and testing are usually dictated by local legislation. Exterior and interior visual inspections, examination of the shell and weld seams, and testing of the SRV, should all be conducted at scheduled intervals.

Additional Resources

https://en.wikipedia.org/wiki/Pressure_vessel

https://www.sciencedirect.com/topics/chemistry/pressure-vessel

https://www.wattco.com/2015/02/what-is-a-pressure-vessel/

Encyclopedia - saVRee