Ammonia Silo Piping

October Blog Entry

Author: Jarod Hansen

Ammonia Silo Piping

 

Refrigerant cooled silos are a very common heat exchanger in the food processing industry.  Typically the liquid product in the silos is cooled by either water, glycol, or ammonia.  A large portion of industrial processing plants utilize ammonia as their refrigerant.  Some silos are used for maintaining product temperature while others need to lower product temperature which has a large impact on refrigerant equipment and line sizing.

There are two primary ways to feed refrigerant to ammonia jacketed silos, direct expansion or flooded.  Unlike flooded systems, direct expansion systems do not require a surge drum; however, the use of thermal expansion valves can be difficult to maintain temperature and need to be replaced over time.  However, the exact life span of the valves varies and manufacturers don’t often provide a set life expectancy on their equipment.  From the valve manufacturers Carlson and Stewart Refrigeration has consulted with, it could range from 3 to 10 years.  The newer (more expensive) electronic thermal expansion valves are more reliable and do provide better control of maintaining temperature.

The majority of new silo installations utilize a flooded ammonia arrangement.  The use of a flooded surge drum provides a more consistent cooling method.  It helps smooth out the rapid changes in temperature of the product by only worrying about maintaining a consistent liquid level in the surge drum.  Another advantage is the pressure drop on the system.  Mechanical thermal expansion valves require a large pressure drop to operate which in some cases may require the entire ammonia system to run a higher head pressure than otherwise necessary.  Lastly, a flooded arrangement doesn’t require the need to maintain a superheat.  Many technicians and operators can attest to the challenges of finding the “sweet” spot when trying to locate the correct placement of the temperature probe on the suction line.        

One change in the industry over time has been the silo jacket pressure rating.  On older silos many heat exchangers have a pressure rating of 100 psig.  Since the silo jacket is an ASME rated vessel, in a flooded application the relief valves must be selected to the lowest pressure of the isolatable system.  This is why the rated 100psig heat exchanger pressure can pose a problem for designers and plants selecting relief valves.  The 100 psig rating has been an issue over the years for plants because during clean out cycles the inside of the vessel is flushed with very warm water and if the ammonia is not evacuated soon enough in the vessel the temperature in the ammonia jacket and surge will rise quickly.  If the ammonia temperature rises above 63°F the pressure relief valves will lift and the plant will have to deal with an ammonia release to atmosphere.  The industry has essentially put a stop to rating these vessels at such a low pressure.  IIAR2-2014 calls out for the minimum vessel pressure rating to be 250 psig, which is a corresponding saturated temperature of 115°F.

CSR has piped many ammonia refrigerated silos over the past 83 years.  In equipment replacement scenarios, CSR would recommend updating mechanical thermal expansion valves with electronic valves, as well as insulating any suction piping that was not previously insulated.  In new silo piping scenarios, CSR engineers recommend installing a dedicated pumpout line in flooded applications to speed up the time production has to clean provided there are measures to handle this influx of liquid refrigerant in the compressor room.  This also helps with reducing the possibility of over pressurizing the circuit and popping the relief valves.  We would also recommend having the silo manufacture install flanged connections on the ammonia circuits in order to reduce leakage.  Without the flanges, the silo connection point is typically threaded stainless steel which is an easy place for a leak to occur because typically a dissimilar metal flange is attached, sometimes even threaded, onto the connection which expands at a different rate.