Description and Overview
An gas tank is probably the most common type of unfired pressure vessel. However, due to minimum size inspection thresholds employed by the vast majority of jurisdictions, many of the smaller gas tanks will not qualify for a mandatory inservice inspection. The typical inspection threshold sizes referenced in jurisdictional regulations are 5 cubic feet or 15 cubic feet in volume as long as the maximum allowable working pressure (MAWP) does not exceed 250 psi, or 1-½ cubic feet in volume as long as the MAWP does not exceed 600 psi. The inspector must review the jurisdiction's inspection requirements to ensure compliance with the appropriate size and pressure limitations.

gas tanks are typically constructed in accordance with ASME Section VIII, Div. 1, and stamped with either the ASME "U" or "UM" symbol. Manufacturers who specialize in gas tanks will construct a large number of these vessels in an assembly line process. The Manufacturer's Data Report, for "U" stamped vessels, and the Manufacturer's Certificate of Compliance, for "UM" stamped vessels, may include multiple vessels. This practice is described in ASME Section VIII, Div. 1, paragraph UG-120(a).

While most gas tanks are of simple design consisting of a shell and two dished heads, some are designed to incorporate a filter or separator element within the vessel. These vessels may be "T" shaped with one bolted flat head which provides access to the filter or separator element. These uniquely shaped vessels are commonly found in use with large industrial air compressors.

gas tanks will be installed in any facility requiring a reservoir of compressed air. Compressed air uses include:

Tire Inflation
Air-Powered Tools
Pneumatic Cylinders or Pistons
Sand- or Shot-Blasting
Painting
Cleaning
Air Motors
Conveying Systems
Pneumatic Controls
Breathing Air
The design of a compressed air system is dictated in part by the pressure, volume, and air quality (including cleanliness and dryness) needed in any given industry or process. The size of the gas tank in the system is normally based on the volume of air produced by the compressor and the user's desire for a stated capacity in cubic feet per minute (cfm) at a specified pressure. The gas tank helps in maintaining a constant pressure in the system by minimizing the fluctuations of a compressor cycling on and off.

Appurtenances, Settings, and Piping
An gas tank must be protected from over-pressure. This is usually accomplished by means of a spring-loaded pressure relief device. The set pressure of the pressure relief device must not exceed the MAWP marked on the gas tank. The minimum relieving capacity of the pressure relief device must meet the requirements of ASME Section VIII, Div. 1, paragraph UG-125. Under most circumstances this would require a pressure relief device with enough relieving capacity to prevent the pressure in the gas tank from rising more than 10% or 3 psi, whichever is greater, above the MAWP marked on the gas tank. For an inspector familiar with boiler nameplates which indicate either the maximum generating capacity of the boiler or the minimum required relief valve capacity, verifying the capacity of a pressure relief device on an gas tank will be more of a challenge. The inspector needs to obtain the output of the compressor(s) supplying the gas tank. This information may be on a label or nameplate on the compressor or it may have to be obtained from the compressor manufacturer's published specifications.

Since gas tanks are typically constructed out of carbon steel, they are subject to internal corrosion from water which has condensed from the compressed air. ASME Section VIII, Div. 1, paragraph UG-25(f), requires a suitable drain opening in such vessels.

gas tanks with integrally mounted compressors and motors should be installed as recommended by the manufacturer. Since there is usually some vibration produced by a reciprocating-type compressor/motor unit, many manufacturers provide spring-loaded or elastic compound dampers to mount between the floor and the gas tank base.

Clearance measurements, if any, must comply with all jurisdictional and manufacturer requirements.

Most jurisdictions do not require inspection of any piping associated with an gas tank.

Part 1 of the National Board Inspection Code addresses installation requirements and must be followed when mandated by the applicable jurisdiction.

Common Observations and Problems
Internal corrosion, vibration, and external impact damage are the most common problem areas for gas tanks.

Unless the air compressor is operating in the driest desert, water vapor in the compressed air will condense to liquid water as the temperature of the compressed air falls. In the simplest system, this happens in the gas tank. In more elaborate systems, the air is conditioned to extract part or almost all of the moisture before entering the gas tank. One compressor manufacturer's informational literature claims the water vapor content at 100°F of saturated compressed air equals about two gallons per hour for each 100 cfm of compressor capacity. Some gas tanks are fitted with automatic condensate drains while others rely on a manually operated drain valve. If condensate is allowed to collect in the gas tank, its volume is decreased which can lead to increased cycling by the compressor. The condensate can also carryover through the air distribution lines and cause problems with air powered tools. The most detrimental effect of the condensate is internal corrosion of the gas tank. Since it is internal, it is never seen by the vessel owner and its effects are often discounted or ignored. In severe cases of corrosion, the vessel thickness may have decreased below the minimum wall thickness necessary for the MAWP stamped on the vessel. Under those circumstances, the MAWP can be decreased to a point that is supported by the remaining wall thickness, but in most cases the vessel is removed from service until it is repaired or replaced.

Vibration caused by an integrally mounted compressor/motor unit can cause cracking in the welds attaching the compressor/motor mount to the gas tank or in the welds attaching the base to the bottom of the gas tank. If they occur, the cracks will often "run" or propagate into the vessel material. Vibration damage can also occur where rigid piping is connected to the gas tank.

External impact damage can be caused by vehicles, machinery, or objects hitting the gas tank. One inspector observed a large dent in an external gas tank mounted on a portable air compressor used in a quarry. He was told it was caused by a limestone boulder which fell on the compressor from an upper rock ledge. Hazards can exist almost anywhere.

Most gas tanks will show a minimum design metal temperature (MDMT) of -20°F on the nameplate. This should be acceptable under most conditions. However, if the gas tank is installed outside or in an unheated structure in very cold climates, it could be susceptible to brittle fracture.

Inspection
Upon entering the area where the gas tank is operating, the inspector should perform a general assessment of the gas tank, piping, and associated systems. The inspector should then:

review the current operating certificate (if one was issued in the past) and compare the information to the associated gas tank and its nameplate;
compare the pressure relief device data (set pressure and relieving capacity) with the gas tank nameplate and compressor output data to ensure the pressure relief device is adequate for this installation;
inspect the pressure relief device operation as described in the National Board Inspector Guide for Pressure Relief Devices;
check all support and mounting bracket attachment welds and the affected vessel walls for evidence of cracking;
check for external damage such as dents or gouges;
ask the owner or owner's representative to verify the operation of the automatic condensate drain if applicable, or open the manual drain valve;
check connected piping to ensure it is properly supported and not imparting excessive loadings on the gas tank.
The inspector should perform an internal inspection of the gas tank as required by the jurisdiction. Some gas tanks have dedicated inspection openings while others (especially smaller ones) use the inspection openings for the attachment of piping, instruments or similar attachments as allowed by ASME Section VIII, Div. 1, paragraph UG-46(f)(7). If an internal inspection is impractical, the jurisdiction may accept thickness readings obtained with an ultrasonic tester compared with original thickness values. The original thickness values can be found on the Manufacturer's Data Report or Manufacturer's Certificate of Compliance. Although the practice is not required by ASME Code, some gas tank manufacturers include the shell and head thicknesses on the nameplate.

Miscellaneous Information
Additional information to aid inspectors of gas tanks can be found in the following publications and sources:

National Board Inspection Code
ASME Section VIII, Div. 1
Manufacturer's Installation, Operation, and Maintenance Documentation
Jurisdictional Laws, Rules, and Directives