Over the years, ammonia piping design has been modified so that much of the piping throughout the plants is now located on the rooftop. This replaces the older design philosophy of running pipes directly within the plant, through the various refrigerated spaces, processing areas and traffic hallways. Older plants quite often had a hung ceiling in which the piping was located. Access was difficult and rarely was there enough headroom in which to stand up straight. Flooring and lighting in that space were typically minimal.
Piping outside of the building and on the roof has reduced the risk of ammonia release within the facility and lessened the possibility of plant personnel contact directly with ammonia. It also has improved the hazards of working on ladders, platforms and lifts to service valves, pressure regulators and other control components at or near the ceiling within an operating plant.
All of this has been a good thing. Piping on the rooftop permits service and repairs while standing on firm footing and working at a convenient level. Penthouses are readily serviced because valve stations are typically outside and adjacent to the penthouse, and there is not always the necessity of entering the extreme cold of blast freezer rooms.
However, although rooftop piping is a good thing, it has brought along a number of problems. These must be addressed to provide the most reasonable, economical and safe service and maintenance areas while maintaining adherence to the ASHRAE 15 and IIAR 2 codes. The OSHA (PSM) and EPA (RMP) program requirements also must be followed where they are applicable.
This column will discuss some of the more common rooftop problems and provide recommended actions for improvement.
Rooftop Access. Many plants have only one straight, vertical steel ladder (with rungs, not steps) fastened directly to the wall within or outside the building. Access is then at the edge of the roof or through a trap door that also may be at the edge. Roof edges are dangerous at these points; dangers also exist where equipment to be serviced is at or near the edge. Wet, rainy surfaces, ice and snow, poor or no lighting and sloped metal roofing portend "at risk" situations.
Safe access should include at least one steel main staircase, with alternate ways of escape in the case of an emergency incident. Good lighting over the roof area, especially at the service locations, is required. Sturdy railings are a must at the critical edge locations to provide for a safe working environment. Easy, safe access also will permit others -- supervisors, inspectors and technicians -- to get up on the roof and assist the operators in maintaining the system integrity.
Piping. Unfortunately, the roofs on a number of plants that have had major expansions over the years are covered with refrigeration (as well as process) pipes, running in a haphazard manner. These pipes often block direct pathways to the various valve stations that are the primary areas of operator concern. Repeated climbing over 4' high stacks of piping will certainly take its toll of damage on any insulation, breaking the outer protective covering and then voiding the critical vapor barrier. Even worse are those runs that are too high to readily climb over but that have about 24" of roof clearance. Quite often, the regular passageway is to lie on the roof and wiggle under the pipe -- not much fun in the rain or snow.
The best way to eliminate this concern is to map where the operators need to go and then place galvanized catwalks with bridges over intervening pipes. This also permits other personnel to get to and do necessary maintenance on the roof.
Insulation. The quality of the insulation on refrigerated pipes must be monitored. Insulation that is cold, wet and icy on the underside of the pipe is of less than no value. The old adage, "Wet insulation is no insulation," still applies. Refrigeration system capacity is lost through the ice buildup and melting, and pipes under such conditions are subject to serious corrosion.
The solution is to strip the old insulation; check the pipe for corrosion; if adequate, clean the pipe; paint with corrosion-inhibiting paint; and re-insulate, ensuring that the vapor barrier is continuous. If the failed insulation has led to pipe failure, you may need to replace the entire piping system.
Piping Supports. Another problem area is the piping supports for long runs of pipe. Depending on the corrosive properties of the geographic location, with time, the supports may rust and totally disintegrate, leaving the pipe unsupported over long stretches. This results in drooping, which may cause pockets of liquid, especially in hot gas defrost lines that can cause liquid hammer in the coils during defrost.
Valve Stations. The valve stations are located above every cooler/freezer evaporator coil or adjacent to the penthouses. These are the prime target of the system operators, who periodically take pressure and temperature data and make the minor adjustments necessary to tune the system. There may be six or more valves, manual or solenoid, as well as pressure regulators and hot gas defrost controls at each station.
Quite often, these valves are not insulated, and the insulation on the piping stops at the valve station. Unfortunately, the end of the insulation often is not vapor sealed, allowing moisture to wick into the insulation and destroying its effectiveness.
There are also issues of welded connections not painted, valve stems rusted, valve caps missing, pressure gauges with broken lenses, and an occasional electrical junction box without its cover.
It just takes common sense to ferret out these types of issues and take corrective action as required. Easy access to the roof will allow more eyes to survey the condition of the equipment and to set up the maintenance necessary to retain the mechanical integrity of the plant rooftop. A painter with a wire brush and a high-quality rust-preventive paint can work wonders as he periodically visits the rooftop, enjoying the view and solitude while preventing the system valves and piping from rusting into disaster.
Relief-Valve Venting. The vent piping from most system relief valves is found on the roof but a reasonable percentage does not make it more than just above the roofline. Code requirements state that the vent outlet must be at least 15' above ground (rooftop) level and positioned so that a release from the valves is not directed toward any ventilation inlet or location that may normally be occupied by operating personnel. Look around your plant, read the ASHRAE 15 Code and make sure that your personnel are not at risk if a release occurs.
Evaporative Condensers. A number of problems stem from the evaporating condensers, which usually are located on the roof and unfortunately quite close to the roof edge. Personnel safety is a major concern because condenser service requires climbing to the top of the unit to check and clean the mist eliminators and spray nozzles. Permanent ladders and catwalks are required safety items; old, rotting wooden ladders are no longer in vogue.
Because recirculated water (typically at 80 to 85oF [26 to 29oC]) is a major component of these systems, care must be taken to ensure proper water treatment suitable for your plant location. It should control mineral content (to avoid tube scaling); control pH; and kill virtually all bacteria, molds and slime associated with the nice warm pan water in the condenser. It may be surprising to know that almost every pond, stream and evaporative condenser in the United States may contain some Legionella bacteria. When biological treatment is not a regular procedure for the condensers, then your operators, all of your employees, and the general public who may come in contact with the mist from the condenser are at risk. Legionnaires' disease is a type of pneumonia with an extremely high fatality rate. Don't let the pretty long green streams of slime dangling from the condenser mesmerize you. Get your water treatment specialist in and get rid of the bacteria -- and everything else living in that condenser.
Because they function by spraying water over the coils, condensers tend to be moist all over. They may leak from the side-panel joints and, when untended, will eventually rust out the steel beams on which they are mounted. This rust also may creep down any vertical supports. The support structure will need to be replaced if not maintained. On new equipment, use galvanized steel or at least paint them before installation -- and maintain them to help prevent rust.
Maintaining refrigerant equipment and piping systems on the roof of a plant is not rocket science. Almost anyone can look around, see the most obvious shortcomings and direct what needs to be done -- but, first, easy and safe rooftop access is the key. There is a lot of expensive, critical equipment on the roof. Don't ignore it, or it may just go away.