Selecting systems to improve function, safety and efficiency
By John Karpus Applications Specialist, Concoa
A 12-cylinder acetylene manifold with flashback arrestors.
The 632 Switchover is appropriate as a pressure differential switchover for an industrial gas application.
Three options shown here represent proper locations for flashback arrestors in manifold installations.
A call came in recently from a Midwest distributor who was looking for manifold options for a new fuel gas system. His customer had given him the opportunity to demonstrate an alternate fuel gas, and had seen value in its safety and performance.
The distributor’s customer was ready to change to the new fuel gas, but the sticking point was that the facility had existing oxygen and acetylene manifold and piping, and didn’t want to replace either of them.
Prevailing wisdom suggested recommending that a safety inspection be performed on both of the customer’s systems as well as opening one of the drip leg ports to inspect for moisture, rust or anything else that shouldn’t be there. Also, a basic safety checklist was reviewed, and the use of a switchover system was proposed.
The distributor called back several days later. He confirmed that some contamination was found at one of the end locations in the fuel gas piping, and he and his customer decided to isolate one of the legs of the system, and blow the line out with nitrogen. T
he customer soon saw, in dramatic fashion, that there was a significant problem. The amount of contamination and its location were alarming, and the distributor said it took about an hour to be able to see across the shop again.
The acetylene manifold that was in use, it turned out, was at least 10 years old. The main line had a liquid flashback arrestor, but the customer had never installed any flashback arrestors or check valves at the end points. The liquid flash arrestor was empty, and apparently had been for some time.
The contamination that sent the alarming cloud through the shop was the leftover carbon and soot from the years of acetylene that had burned back in the pipeline and was finally discovered.
The customer now was interested in a new fuel gas manifold, piping, and new point-of-use equipment complete with flashback arrestors and check valves. He also was interested in replacing his old oxygen manifold and its point-of-use regulators with new equipment that included flashback arrestors and check valves at every station.
Another issue arose that also was addressed.
When the old manifold systems were installed, the lines were dead-ended at the last drop. The two new systems were looped to insure that the pressure inside would equalize and that no operator would be”starved” of product in a high-demand situation.
This story ended with the distributor walking out of the customer’s location with a big order for all the equipment required to update both the oxygen and fuel gas systems.
Safety and performance audits
At last report the end-user was happy with his new equipment. It is worth noting, however, that had the distributor’s sales representative offered to start with a safety check list or inspection, the big issues could have been discovered and addressed earlier in the process.
The performance of the new system combined with the correct placement of flashback arrestors and check valves to provide provided the solution. The situation could have turned out much differently had there not been the opportunity to inspect and review the old systems.
The point of the story comes down to this: One of the most valuable assets you offer a customer is your knowledge. Communicating practical – and theoretical -- information has untold advantages. Conducting a safety and performance audit (SPA), for example, can insure that your customer’s shop will be a safer place to work and, in turn, a more profitable business.
A safety and performance audit is a valuable second or third step sales tool that can be used to open new accounts. It might leave potential customers wondering why their current supplier didn’t offer such service and, in an existing account, the audit usually is perceived as a value-added service that could help to limit business liability due to injury or loss.
A safety and performance audit should address the process the customer is using, and is an opportunity to recommend ways to improve efficiencies. It also is an opportunity to establish in-house training, such as loss prevention and safety, to fulfill training requirements that many businesses have.
Understand Basic Gas Delivery Principles
There are three types of changeover manifold systems available for gas distribution, manual, semi-automatic, and automatic.
With manual systems, multiple cylinders feed into a manifold that is controlled by a regulator. The manifold regulator lowers the pressure to supply gas to the in-house piping that delivers it to the points of use.
Semi automatic switchovers typically are pressure differential systems that use two input regulators. Typically, a knob on one input regulator must be rotated to point towards the side that supplies it.
The output for these initial regulators are tied together, but the pressure for one of them is set 20 psi lower than the other.
As the pressure from the primary cylinder decreases and reaches the set point of the second regulator, the seat on the first regulator closes. When that happens, the seat on the second regulator opens to supply the system. A line regulator adjusts and maintains the final delivery pressure in the system. .
A fully automatic system makes the cylinder change from side to side without input from an operator. The only job the operator must complete is to replace empty cylinders.
In most cases, these manifold systems are modular and can be upgraded by adding extension kits, and they are flexible so they can meet demands for increased or decreased gas volume with by adding or removing cylinders at the manifold.
Appropriate systems and equipment
National Fire Protection Agency (NFPA) Standard 51 recommends that welding and cutting equipment be inspected visually on a daily basis or prior to being used, and leak-checked with a bubble solution monthly.
It is critical to look for deficiencies and oversights in the design and installation of existing gas delivery systems. The location of the correct protective equipment, such as shutoff valves, check valves, flashback arrestors, drip legs and relief valves is detailed in National Fire Protection Agency Standard 51.
Three different examples in the standard illustrate a typical manifold system and the proper location of these devices. The standard requires that flashback arrestors be located in the main supply line, (Figure 1), at the head of each branch line (Figure 2), or at each location where fuel gas is withdrawn (Figure 3).
Only a shutoff valve is required at a fuel gas station outlet when a flashback arrestor is located , there. . When a flashback arrestor is located as shown in Figures 1 and 2, check valve protection of the fuel gas supply also must be provided at the station outlet to prevent oxygen from flowing into the fuel gas system.
It is important to note the section of Standard 51 that details the materials that are to be used in the construction of the manifold and piping system to ensure materials compatibility. Copper is never to be used for acetylene applications. Piping for fuel gases such as acetylene, LP, and MAPP must be steel.
Everything should be cleaned for oxygen service. Oxygen manifolds must be separated a minimum distance of 20 feet from fuel gas cylinders, flammable material, or an electrical source capable of producing an ignition, or by a barrier of non-combustible material at least five feet high and with a minimum fire-resistance rating of one half of an hour.
The quantity of fuel gas allowed on a manifold inside a building is limited to 3000 cu. ft. of acetylene, or 735 lbs. (water capacity) of LP gas or MAPP. The quantity of oxygen gas allowed on a manifold inside a building is limited to 6500 cu. ft. for high-pressure cylinders and 12000 cu. ft. for low-pressure cylinders.
Systems that require cylinder volume higher than that limit must be located outside. Both fuel and oxygen systems require that a pressure- relief device be installed in the line. A relief device on a manifold regulator does not satisfy this requirement.
A readily accessible gas valve must be provided to shut off the gas supply to buildings in case of emergency, and a shut -of f valve also must installed in the discharge line from the manifold source of supply. Signs that clearly establish the location and identity of shutoff valves also must be provided.
Station outlets and protective equipment
An approved shutoff valve is required at each outlet, and must be located on the upstream side of other station outlet equipment. An approved check valve also must be installed at each station outlet, either upstream or downstream of the shut-off valve.
When a flashback arrestor is located at a fuel gas station outlet as in Figure 3, the only other device required is a shutoff valve. If the station outlet is equipped with a detachable regulator, the standard says that the outlet must terminate in a union connection that complies with CGA pamphlet E-3.
Additionally, the standard requires that station outlets be equipped with a detachable dust cap that must be secured in place except when a hose, regulator, or piping is attached to the outlet.
As many as four torches can be supplied from one drop, provided that each outlet is equipped with a shut off valve, flashback arrestor and a check valve. The only limitation is that the capacity of one torch cannot exceed 15 cu. ft. per hour of acetylene, LP gas or MAPP, and 50 cu. ft. per hour of natural gas, methane or hydrogen.
Understanding the basic elements and principles of gas delivery and how to select appropriate systems and equipment goes a long way for distributors today as system needs and requirements continually evolve and change.
Want to use this article? Click here for options!
© 2013 Penton Media Inc.