(m) Description and concerns of specific
types of pressure vessels.
(1) Compressed air
vessels. The following applies to compressed air vessels:
(A) Considerations of concern include
temperature variances, pressure limitations, vibration, and condensation. Drain
connections shall be verified to be free of any foreign material that may cause
plugging; and
(B) Inspections of
compressed air vessels shall consist of the following:
(i) Welds. Inspect all welds for cracking or
gouging, corrosion, and erosion. Particular attention shall be given to the
welds that attach brackets supporting the compressor. These welds may fail due
to vibration;
(ii) Shells and
heads: externally, inspect the base material for environmental deterioration
and impacts from objects. Hot spots and bulges are signs of overheating and
shall be noted and evaluated for acceptability. Particular attention shall be
paid to the lower half of the vessel for corrosion and leakage. For vessels
with manways or inspection openings, an internal inspection shall be performed
for corrosion, erosion, pitting, excessive deposit buildup, and leakage around
inspection openings. Ultrasonic thickness testing (UT) may be used where
internal inspection access is limited or to determine actual thickness when
corrosion is suspected;
(iii)
Fittings and attachments. Inspect all fittings and attachments for alignment,
support, deterioration, damage, and leakage around threaded joints. Any
internal attachments such as supports, brackets, or rings shall be visually
examined for wear, corrosion, erosion, and cracks;
(iv) Operation. Check the vessel nameplate to
determine the maximum allowed working pressure and temperature of the vessel.
Ensure the set pressure of the safety valve does not exceed that allowed on the
vessel nameplate and determine that the capacity of the safety valve is greater
than the capacity of the compressor. Ensure there is a functioning manual or
automatic condensate drain; and
(iv) Quick closure attachments. Filter-type
vessels usually have one quick-type closure head for making filter changes, see
NBIC Part 2, 2.3.6.5;
(2) The following shall apply to pressure
vessels with quick-actuating closures:
(A)
Due to the many different designs of quick-actuating closures, potential
failures of components that are not specifically covered shall be considered.
The scope of inspection shall include areas affected by abuse or lack of
maintenance and a check for inoperable or bypassed safety and warning
devices;
(B) Temperatures above
that for which the quick-actuating closure was designed can have an adverse
effect on the safe operation of the device. If parts are found damaged and
excessive temperatures are suspected as the cause, the operating temperatures
may have exceeded those temperatures recommended by the manufacturer. Rapid
fluctuations in temperatures due to rapid start-up and shutdown may lead to
cracks or yielding caused by excessive warping and high thermal stress. A
careful observation shall be made of the condition of the complete
installation, including maintenance and operation, as a guide in forming an
opinion of the care the equipment receives. The history of the vessel shall be
established, including: year built, materials of construction, extent of post
weld heat treatment, previous inspection results, and repairs or alterations
performed. Any leak shall be thoroughly investigated and the necessary
corrective action initiated;
(C)
Inspection of parts and appurtenances. Seating surfaces of the closure device,
including but not limited to the gaskets, 0-rings, or any mechanical
appurtenance to ensure proper alignment of the closure to the seating surface,
shall be inspected. This inspection can be made by using powdered chalk or any
substance that will indicate that the closure is properly striking the seating
surface of the vessel flange. If this method is used, a check shall be made to
ensure that:
(i) Material used shall not
contaminate the gasket or material with which it comes into contact;
and
(ii) The substance used shall
be completely removed after the examination;
(D) The closure mechanism of the device shall
be inspected for freedom of movement and proper contact with the locking
elements. This inspection shall indicate that the movable portions of the
locking mechanism are striking the locking element in such a manner that full
stroke can be obtained. Inspection shall be made to ensure that the seating
surface of the locking mechanism is free of metal burrs and deep scars, which
would indicate misalignment or improper operation. A check shall be made for
proper alignment of the door hinge mechanisms to ensure that adjustment screws
and locking nuts are properly secured. When deficiencies are noted, the
following corrective actions shall be initiated:
(i) If any deterioration of the gasket,
O-ring, etc., is found, the gasket, 0-ring, etc., shall be replaced
immediately. Replacements shall be in accordance with the vessel manufacturer's
specifications;
(ii) If any
cracking or excessive wear is discovered on the closing mechanism, the owner or
user shall contact the original manufacturer of the device for spare parts or
repair information. If this cannot be accomplished, the owner or user shall
contact an organization competent in quick-actuating closure design and
construction prior to implementing any repairs;
(iii) Defective safety or warning devices
shall be repaired or replaced prior to further operation of the
vessel;
(iv) Deflections, wear, or
warping of the sealing surfaces may cause out-of-roundness and misalignment.
The manufacturer of the closure shall be contacted for acceptable tolerances
for out-of-roundness and deflection; and
(v) The operation of the closure device
through its normal operating cycle shall be observed while under control of the
operator. This shall indicate if the operator is following posted procedures
and if the operating procedures for the vessel are adequate;
(E) Gages, safety devices, and
controls. The required pressure gage shall be installed so that it is visible
from the operating area and located in such a way that the operator can
accurately determine the pressure in the vessel while it is in operation. The
gage dial size shall be of such a diameter that it can be easily read by the
operator. This gage shall have a pressure range of at least one and a half
(1.5) times, but not more than four (4) times, the operating pressure of the
vessel. There shall be no intervening valve between the vessel and
gage;
(F) The pressure gage shall
be of a type that will give accurate readings, especially when there is a rapid
change in pressure. It shall be of rugged construction and capable of
withstanding severe service conditions. Where necessary, the gage shall be
protected by a siphon or trap;
(G)
Pressure gages intended to measure the operating pressure in the vessel are not
usually sensitive or easily read at low pressures approaching atmospheric. It
may be advisable to install an auxiliary gage that reads inches of water and is
intended to measure pressure from atmospheric through low pressures. This
ensures that there is zero pressure in the vessel before opening. It would be
necessary to protect the auxiliary low-pressure gage from the higher operating
pressures;
(H) Provisions shall be
made to calibrate pressure gages or to have them checked against a master gage
as frequently as necessary;
(I) A
check shall be made to ensure that the closure and its holding elements must be
fully engaged in their intended operating position before pressure can be
applied to the vessel. A safety interlock device shall be provided that
prevents the opening mechanism from operating unless the vessel is completely
depressurized; and
(J)
Quick-actuating closures held in position by manually operated locking devices
or mechanisms, and which are subject to leakage of the vessel contents prior to
disengagement of the locking elements and release of the closure, shall be
provided with an audible and/or visible warning device to warn the operator if
pressure is applied to the vessel before the closure and its holding elements
are fully engaged, and to warn the operator if an attempt is made to operate
the locking device before the pressure within the vessel is released. Pressure
tending to force the closure clear of the vessel must be released before the
closure can be opened for access; and
(3) Inspection of Pressure Vessels for Human
Occupancy (PVHOs). The following shall apply to the inspection of PVHOs:
(A) General and operational. PVHOs shall be
constructed in accordance with ASME PVHO-1. This code adopts ASME BPV Section
VIII and therefore the vessels shall bear a "U" or "U2" ASME designator.
Inspections should be conducted using ASME PVHO-2 for reference;
(B) Cast and ductile iron fittings are not
allowed;
(C) Due to the human
occupancy element, a person shall be in attendance to monitor the PVHO when in
operation, in the event there is an accident;
(D) The installation shall be such that there
is adequate clearance to inspect it properly. In some applications, such as
underground tunneling, it may be impossible to perform a complete external
inspection;
(E) Internal
inspection. Where existing openings permit, perform a visual internal
inspection of the vessel. Look for any obvious cracks and note areas that are
subject to high stress such as welds, welded repairs, head-to-shell
transitions, sharp interior corners, and interior surfaces opposite external
attachments or supports. The vessel shall be free of corrosion, damage, dents,
gouges, or other damage. All openings leading to external fittings or controls
shall be free from obstruction. All exhaust inlets shall be checked to prevent
a chamber occupant from inadvertently blocking the opening;
(F) External inspection. The inspector shall
closely examine the external condition of the pressure vessel for corrosion,
damage, dents, gouges, or other damage. The lower half and the bottom portions
of insulated vessels shall receive special focus, as condensation or moisture
may gravitate down the vessel shell and soak into the insulation, keeping it
moist for long periods of time. Penetration locations in the insulation or
fireproofing such as saddle supports, sphere support legs, nozzles, or fittings
shall be examined closely for potential moisture ingress paths. When moisture
penetrates the insulation, the insulation may actually work in reverse, holding
moisture in the insulation or near the vessel shell. Insulated vessels that are
run on an intermittent basis or that have been out of service require close
scrutiny. In general, a visual inspection of the vessel's insulated surfaces
shall be conducted once per year. The most common and superior method to
inspect for suspected corrosion under insulation (GUI) damage is to completely
or partially remove the insulation for visual inspection. The method most
commionly utilized to inspect for GUI without insulation removal is by X-ray
and isotope radiography (film or digital), or by real time radiography,
utilizing imaging scopes and surface profilers. The real-time imaging tools
will work well if the vessel geometry and insulation thickness allows. Other
less common methods to detect GUI include specialized electromagnetic methods
(pulsed eddy current and electromagnetic waves) and long-range ultrasonic
techniques (guided waves). There are also several methods to detect moisture
soaked insulation, which is often the beginning for potential GUI damage.
Moisture probe detectors, neutron backscatter, and thermography are tools that
can be used for GUI moisture screening. Proper surface treatment (coating) of
the vessel external shell and maintaining weather-tight external insulation are
the keys to prevention of GUI damage;
(G) Inspection of parts and appurtenances
(e.g., piping systems, pressure gage, bottom drain). As stated above, cast iron
is not allowed on PVHOs and shall be replaced with parts fabricated with other
suitable materials, in accordance with ASME BPVG Section II. If valves or
fittings are in place, check to ensure that these are complete and functional.
The inspector shall note the pressure indicated by the gage and compare it with
other gages on the same system. If the pressure gage is not mounted on the
vessel itself, it shall be ascertained that the gage is installed on the system
in such a manner that it correctly indicates actual pressure in the vessel. The
inspector shall verify that the vessel is provided with a drain opening. The
system shall have a pressure gage designed for at least the most severe
condition of coincident pressure in normal operation. This gage shall be
clearly visible to the person adjusting the setting of the pressure control
valve. The graduation on the pressure gage shall be graduated to not less than
one and a half (1.5) times the maximum allowable working pressure (MAWP) of the
vessel. Provisions shall be made to calibrate pressure gages or to have them
checked against a standard test gage. Any vents and exhausts shall be piped at
least ten (10) feet from any air intake. Venting shall be provided at all high
points of the piping system;
(H)
Inspection of view ports and windows. Each window shall be individually
identified and be marked in accordance with PVHO-1. If there are any
penetrations through windows, they must be circular. Windows must be free of
crazing, cracks, and scratches. Windows and viewports have a maximum interval
for seat or seal inspection and refurbishment. Documentation shall be checked
to ensure compliance with PVHO-2, Table 2-4.3- 1, Table 2-4.3-2 (see Exhibit
1); and
(I) Inspection of pressure
relief devices. Pressure relief devices must have a quick opening manual
shutoff valve installed between the chamber and the pressure relief device,
with a frangible seal in place, within easy access to the operator. The
pressure relief device shall be constructed in accordance with ASME BPVC
Section VIII. The discharge from the pressure relief device must be piped
outside to a safe point of discharge. Rupture disks may be used only if they
are in series with a pressure relief valve, or when there is less than two (2)
cubic feet of water volume. Verify that the safety valve is periodically tested
either manually by raising the disk from the seat or by removing and testing
the valve on a test stand.