To assure the safe operation of the High Pressure synthesis vessels in a urea plant the sound condition of the loose liners is very important. The soundness of these liners should be inspected during maintenance turnarounds. And its integrity should be continuously checked during operation of the plant via a proper leak detection system. Best practice is to appoint one person to be responsible for the system plus shift responsibility for frequent checking.
A leak detection system with a short response time is important to avoid the formation of solids behind the liner. To wait until one sees fumes from the leak detection tubes is in my view not a proper leak detection system.
Assure yourself that all leak detection holes communicate, be aware that multi layer carbon steel vessels have weep (ventilation) holes which are not to be confused with the leak detection holes.
Regarding the use of steam in a leak detection system different views exist in the industry. Be aware that the use of steam in a leak detection system can lead to unacceptable risks; namely stress corrosion cracking of the carbon steel pressure wall from the inside or in the liner, thus in an area which is impossible to inspect.
In case of doubt contact your Licensor or equipment manufacturer. For example Stamicarbon has developed a state of the art leak detection system. Please refer to their website for more information.

2009 04 Brouwer UreaKnowHow.com Leak Detection Systems in Urea Plants.pdf