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Why? A Question for All Inspectors


Typical Improper Repairs of Safety Valves


S.F. Harrison Jr
National Board Director of Pressure Relief Department and Lab

52nd General Meeting
May 1983

Category: Design/Fabrication

 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (7 printed pages)

 


 

Recognizing the need for adequate repairs of ASME/National Board stamped safety and safety relief valves, the National Board Executive Committee, in January of 1977, approved procedures for the issuance of the National Board Certificate of Authorization for the use of the valve repair "VR" stamp. The intent of these procedures was to establish realistic standards by which proper repairs could be effected on safety and safety relief valves to restore their performance and condition equivalent to the standards for new valves.

My presentation this morning will review those typical improper repairs being conducted by numerous repair organizations which were the cause of the concern in 1977, as well as today.

Let us first review the definition of a repair to a safety and safety relief valve as described in paragraph 7.1 of the booklet, National Board Authorization to Repair ASME and National Board Stamped Safety Valves and Safety Relief Valves, NB-65{1}. This paragraph defines a repair of a safety and safety relief valve as the replacement, remachining or cleaning of any critical part, lapping of seat and disk or any other operation which may affect the flow passage, capacity, function or pressure retaining integrity. Even the disassembly, reassembly and/or adjustments which affect a valve's function should also be considered a repair. This does not include, however, the testing or adjustments of new valves upon initial installation.

So that we can fully understand the effect of improper repairs, we should review the basic components and function of a safety valve. As I'm sure you are well aware, there are many types and designs of safety valves. Even though individual components may vary by design, the basic function of these parts does not change from one valve to the next. This statement should not be construed to mean that all valves are alike and that if you have the knowledge to repair one valve type, that you can repair all types of valves. Each valve design has its own maintenance requirements and specifications, and should be carefully studied before repair is authorized.

Generally speaking, safety valves will have the following components.

  1. Nozzle -- This is a pressure containing element which constitutes the inlet flow passage.
  2. Seat -- This also is a pressure containing component which makes contact between the fixed and moving portions of a valve.
  3. Guide -- This provides a guiding surface for the disk to insure proper alignment of the seating surfaces.
  4. Stem -- This provides a means to connect the disk to the lifting device.
  5. Spring -- This is used to provide the static force necessary to keep the disk in contact with the seat until the set pressure is reached.
  6. Compression Screw -- This is the external adjustment used to establish the valve's set pressure.

For valves which are to have adjustable blowdowns, it is common to find one or more adjustable rings around either the nozzle and/or the disk.

What are the possible effects of improper repairs to safety valves? The single most important effect is a reduction of the valve's relieving capacity. This, after all, is the primary function of a safety valve to relieve at a sufficient capacity to prevent exceeding a certain value above the maximum allowable working pressure of the vessel. Other effects might be a shift in the valve's set pressure or blowdown outside the specified tolerances of the Code. Operationally the valve's moveable components may hang up, flutter or chatter, and leakage or excessive simmering may be present. The valve's lift might be restricted in some way. All of these items, either individually or collectively, could have an overall effect on the valve's relieving capacity.

Improper repairs to safety valves can be divided into two basic categories. The first category can be attributed to workmanship; the second category can be attributed to the failure to identify and correct problems prior to returning the valve to service.

In the category of improper workmanship, the first that comes to mind is the overall mishandling of the valve and its components. Examples of this might be the overtightening of a valve in a vice during its disassembly. This might result in cracking of body or deforming a part which might result in a misalignment or binding once the valve is reassembled.

Another example of mishandling is supporting the valve by its lift lever. This may cause damage to the valve spindle, such as elongation and bending.

One area of mishandling commonly overlooked is inadequate preparation of the valve prior to shipment. Examples of this include mounting the valve horizontally rather than vertically on a platform, insufficient packing material around the valve, and the absence of covers over the inlet and outlet to prevent the introduction of dirt and debris into the valve internals.

Another area of improper workmanship is in the fabrication of replacement parts. A part should always be fabricated to the original manufacturer's detailed specifications which would include material, dimensions, surface conditions and tolerances. Quite frequently we find this has not been done.

Improper remachining of one worn or damaged component is another area of improper workmanship. The most common problem in this area is that the remachining may place the component's dimensions out of tolerance and may completely alter the intended design characteristics of the component. A typical example may be altering the structure of special disks for high pressure steam service. These disks are designed so that the inlet pressure aids in providing a more effective seal at the seating surface area.

Other examples include machining or lapping the seating surface to a flat condition when the manufacturer's specifications call for a slight bevel, such as 3 to 4 degrees on the seat.

A very common problem encountered is with the use of incorrect springs. Typically, springs are designed to be used over a narrow set pressure range. To insure proper function of the valve, it is essential that the correct spring with a designated spring rate be used for the desired set pressure. Failure to do this may result in restriction of the valve's lift and inability to meet set pressure and blowdown specifications not to mention reduction of the valve's relieving capacity. Another problem encountered is using springs made of a material not suitable for the service conditions of the valve. Failure to use springs which meet the original manufacturer's specifications and following the designated spring charts is the single most contributor to poor valve performance characteristics.

Inadequate preparation of seating surfaces may lead to leakage and possible steam cutting.

Another area of improper workmanship is repair welding by welders where neither themselves nor their procedures have been qualified in accordance with section IX of the ASME Boiler and Pressure Vessel Code. Without insisting this be done, you cannot insure yourselves that the repairs would be adequate and have not affected the parts in an adverse manner such as the inducement of residual stresses caused by the repair welding itself.

The deletion or addition of components should also be considered an improper repair since it would be a deviation from the manufacturer's original design. A horrifying example of this is where a repair organization installed a plug into the valve's nozzle to alleviate the problem of steam leakage due to the severe steam cuts and mushrooming of the nozzle seating area. This was very effective in stopping the leakage; however, the valve's capacity was drastically affected.

Another area of improper workmanship deals with incorrect assembly of the safety valve. In some cases, this can be as equally important as the fabrication of the components themselves. Each manufacturer has a specified procedure by which a valve should be disassembled and reassembled.

Failure to follow these procedures may result in the improper placement and/or orientation of the valve's components. For example, the ring pin which is used to prevent drift of blowdown ring may be installed and bind the ring which would cause misalignment and possible hang up of the disk. General sloppiness in the assembly process may also contribute to valve disfunction and damage such as the cross threading of the nozzle ends of the body which resulted in cracking the body. Along those same lines, poor cleanliness conditions in the assembly area may allow foreign material to be lodged between the disk and seat. This may result in leakage and possible steam cutting of the disk and seating surfaces.

An area that we frequently come across deals with the improper setting of external adjustments to safety valves. It is essential that in order to insure proper operation and function of the valve, a repair organization must have adequate setting facilities. For example, if the setting facility does not include an accumulation vessel of an appropriate size, it may be very difficult to establish the correct blowdown for a valve. Most commonly, if the accumulation vessel is under sized, the valve may give the appearance of having a short blowdown when, in fact, it may have a very long blowdown when tested on a stand with an adequate accumulation vessel. All instrumentation used for setting the valve should be properly calibrated to a national standard. Equally important, the test gauges should be installed in such a way that will provide accurate results.

Using the proper test medium is of equal importance. Valves intended for steam service should be tested on steam. It may not be sufficient to set a valve for steam service on air due to the possible thermal expansion of the components when subjected to steam temperatures.

Since most repair organizations do not have testing facilities capable of measuring the actual capacity of the valve, it is imperative that they follow manufacturer's recommendations when setting safety valves. It is not unlikely for a valve with an adjustable blowdown to be adjusted such that both set pressure and blowdown are within tolerance, but due to the configuration of the blowdown adjustment rings with one another, it is possible that the valve may not get its rated lift and consequently will not relieve at its rated capacity. This is why it is very important for checking lift when setting a valve to insure the lift meets or exceeds the manufacturer's specified minimum lift. Typically, if the valve's lift exceeds the minimum, the valve will have a satisfactory capacity.

Let us now consider the category of improper repairs due to failure to identify and correct deficiencies with one or more components of a safety valve. One of the most commonly overlooked items of this nature is cracked components. 

It is imperative that a repair organization incorporates nondestructive examination into their repair procedures to insure that flaws of this nature do not go undetected.

Another commonly overlooked problem is failure to identify a spring which may be contained in a valve which does not meet manufacturer's specifications. Considering the number of repair organizations in existence today, it is quite possible for a valve to be repaired numerous times after being placed into service and there is no guarantee that at some point the spring, as well as other components in the valve were replaced by an unsuitable component. Along these same lines goes detecting part machining conducted by a previous repairer on the valve's components.

Other commonly overlooked items would be untrue or bent spindles, whether they be gross deformations or slight untrueness, not detectable by the naked eye. Even the slightest amount of untrueness could prevent a valve from attaining its full lift and rated capacity. Worn or galled guiding surfaces when left uncorrected, may cause the valve to bind or hang up.

Valve designs which incorporated a bellows above the disk used to remove the effect of back pressure on the valve's specified set pressure, must be carefully checked to insure the bellows have not been damaged in a way which would allow system back pressure to be felt on top of the disk.

It would be appropriate to ask ourselves at this point, what is the bottom line cause which contributes to allowing improper repairs to continue, and what an organization can do to correct this? We, at the National Board believe that the two most contributing factors are the lack of knowledge of the safety valve design, operation, maintenance procedures and basic repair techniques, as well as failure to have an effective quality control system. Because of this, the "VR" program requires that each repair organization establish and document an in-house training program for all those persons involved in repair of safety and safety relief valves. Each repair organization is required to insure that their personnel making repairs are knowledgeable and fully qualified. To aid repair organizations in this goal, the National Board provides coordinated training courses in Columbus, Ohio, for persons engaged in the repair of safety and safety relief valves. Although these courses do not give specific instruction on basic repair techniques, they do provide the attendees with a basic knowledge to identify and prevent improper repairs, as I have described this morning.

It is essential that each repair organization establish an effective quality control system to insure that valves repaired have been returned to performance and conditions equivalent to the standards for new valves. The National Board provides each repair organization with a guide to aid them in establishing such a quality control system.

By combining the use of competent repair personnel with an effective quality control system, and conducting repairs in accordance with the provisions of the National Board "VR" program, it is possible to overcome those problems I have identified this morning to provide the best assurance that every ASME/National Board stamped safety and safety relief valve repaired will perform as expected when called upon to do so.

I urge each of you to consider the possible effects of allowing improper repairs by unqualified safety valve repair organizations to continue.

I suggest that, if you have not already done so, you initiate legislation within your jurisdictions requiring repairs to safety and safety relief valves be conducted only by qualified repair organizations, such as National Board "VR" stamp holders, to help make this world safer for us all! Thank you for your attention.

FOOTNOTE:
{1} RA-2242 b. and c. of the 1998 National Board Inspection Code

 


 

 

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.

 







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