Double wall piping systems which are fully enclosed, without any possibility of leakage detection, or any method of removing hazardous materials are not particularly helpful. It is always recommended to consider how to detect the leakage or failure, and how to deal with the leakage once a failure has taken place, during the design phase of construction. There are many forms of failure analysis depending on which industry one is accustomed to work in, but they all include the common theme of analysing sources of failure, reducing the risk of failure, and means to deal with the failure once it has occurred. In the case of double wall systems, then that involves essentially how to handle the fluid leaking into the gap between the inner and outer pipes (commonly known as the annular space). As previously mentioned this is an essential part of the design process and integral in the design philosophy of the entire double wall system. Many of the applications will present an obvious solution, whilst other applications may need considerably longer decision making process in order to arrive at the optimal conclusion, and it is often the case that a variety of solutions can be applied, with their own advantages and disadvantages. It should also be noted that specific sectors of industry or company standards may specify the required solution that can be applied. However whenever hazardous substances are involved it is obvious that any failure, and subsequent leakage into the annular space, must not create a new hazardous area. To simplify the design process, the most common forms of double wall leakage detection are :

  1. A ventilation based leakage detection system. In this case a standard ventilation system is applied in an open-ended annular space, with pressure or flow sensors to monitor correct function of the ventilation system. Hazardous fluid detection sensors are used to detect levels of hazardous fluid, thereby triggering any alarm or shut-down measures.
  2. An inerted and sealed detection system, with a pressure or level sensor directly applied to the annular space to detect increase in volume or pressure in the annular space, thereby triggering any alarm or shut-down measures.

When considering the first option then the ventilation system requirements will be heavily dependent on the double wall pipe design and arrangement. This is simple work for an experienced HVAC engineer, however it is often the case that an HVAC engineer is not available for the task, and what is considered a relatively simple task is left up to the piping design engineer designer, who may not have so much experience dealing with ventilation systems. You can therefore find below on this page of the website a simple tool to perform basic ventilation pressure drop and flow calculations for double wall piping, which is based on the AF Pipes double wall pipe support series. All AF Pipe supports are tested for pressure drop characteristics at various flows, and the pressure drop data is provided on the individual support specification sheet for the AF Pipe support of the required dimensions. Correct evaluation of the pressure drop versus flow of the entire double wall system is imperative in order to accurately specify a ventilation fan. We have been witness to many ventilation systems that have been installed with massively oversized ventilation fans, unable to overcome the differential pressure in the system because of the high flow it can produce, therefore inefficiently heating up the surrounding air, as opposed to producing any meaningful flow through the ventilation system, due to too high differential pressure in the piping system. Due to the small pressures involved it can also often be a complex process calibrating pressure sensors to work in the correct range for the alarm and control system.

In the case of option two, then this is a far simpler system to install, without any concerns of ventilation fan sizing, and pressure drops, and it is significantly cheaper due to the reduced complexity of the system. As it is a fully enclosed double wall system, then there are various options available for the leakage detection fluid located in the annular, depending on the intended application. For products where performance is determined by the thermal efficiency, such as heat exchangers, then a thermally conductive liquid could be the preferred fluid. However this can easily lead to design and material complications when used in conjunction with dissimilar materials of the inner and outer pipe, and subsequent corrosion of the materials. Typically, a liquid type detection system will use level switches advantageously providing a simple and robust means of leakage detection.

A pressure leakage detection is also extremely robust, when used in the correct circumstances. There are certainly some situations where due care must be given, particularly when the fluid is exposed to large ambient temperature fluctuations, and a gaseous substance is used as the leakage detection fluid. In this case large fluctuations in the density of the leakage detection gas will consequently lead to pressure fluctuations which unless accounted for in the design of the leakage detection system will be easily mistaken for a leakage in the inner pipe. Use of a simple pressure switch in this situation is not recommended. A pressure transmitter with pre-described level ranges for alarm and shutdown control can be applied successfully in this case, however a slightly less economical solution.

For vacuum insulation double wall piping systems then pressure monitoring is typically supplied as a default item in order to monitor the state of the vacuum as the system is fully enclosed. Many vacuum insulated, or even cryogenic piping systems also include temperature monitoring of the annular space. A loss of vacuum, indicated by the pressure monitoring system could indicate either failure in the outer or inner pipe (or in the case of a high pressure systems the source would be fairly obvious). However, in order to confirm detection and location of the failure, temperature monitoring is often installed along the annular space. This also speeds up location & repair of the fault, to reduce downtime, which is often the absolute priority.

AF Pipe Solutions offers a customised leakage detection and ventilation systems, typically tailored according to customer requirements. Leakage detection is typically either pressure or temperature based for closed systems such as the AF Slim double pipe arrangements, or HC detection when used in conjunction with ventilation systems and the AF Flex double pipe arrangement.