This is surely a prp within the area of equipment maintenance ?? 
Pressure Testing of the Tubes to check for pinholes would be a maintenance pre-requisite carried out during downtime. Normally this would be certificated by a competent contractor.
If you go down the route of over pressure then I would normally have pressure gauges on the cooling system and product. The product pressure must be maintained above that of the cooling system. This is measureable and can be monitored so, depending on how you assess the severity of ammonia getting into product, I suspect this is probably a
CCP.
Regards,
Tony
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