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Calculated verification of safety valve supply lines and blow-off lines

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Calculated verification of safety valve supply lines and blow-off lines

According to AD 2000 Merkblatt A2 as well as ISO 4126 Part 9, pressure losses in the supply line to a safety valve must not exceed 3% of the set pressure at maximum mass flow.

The permissible back pressure in a safety valve blow-off line is specified by the manufacturer and must generally not succeed 10%. Proper function and thus the specified dischargeable mass flow is guaranteed by the manufacturer up to this pressure at the outlet of the safety valve.

Excessive pressure losses in the supply line or excessive back pressure in the blow-off line can cause instable operation of the safety valve. This can lead to violent fluttering and banging of the safety valve, which leads to sharp reduction of the dischargeable mass flow and can lead to the safety valve itself as well as the connected plant parts being damaged by the vibration.

Flow simulation programs such as FLOMASTER® or SuperChems for DIERS® can be used to reliably calculate the supply line pressure losses and the back pressure in the blow-off line for both compressible and incompressible media. If one of these two values is exceeded, various measures can be devised to upgrade the discharge system.

Lines to and behind safety valves should ideally always be as short and as straight as possible. In cases where lines must be longer, such as for structural reasons or due to a collection system, it may be useful to design them in a larger nominal size. Additional equipment can also be used to stabilize the opening behavior, including friction brakes, O-ring vibration dampers or back-pressure compensating metal bellows. We will work with you to find the most intelligent, cost-effective and, above all, safest solution.

In principle, the back pressure is composed of internal back pressure and external back pressure. Limitation of the back pressure downstream from a safety valve normally pertains only to the internal back pressure, since a permanent external back pressure shifts the set pressure upwards but has no influence on the stability of the operation of a safety valve. This no longer applies if a back-pressure compensating metal bellows is used as it also largely compensates the external back pressure and the set pressure is not affected up to a certain level. Our experts are happy to offer you their expertise and experience in this area as well.

For two-phase flow, we can perform pressure loss calculations with the process simulation program ChemCad® and with SuperChems®.

An important prerequisite for determining loss of flow pressure is reliable, temperature-dependent substance data. We like to draw on our extensive substance database, which is based on the DIPPR database. For mixtures of multiple substances, we often use the binary parameters stored in ChemCad to determine more realistic substance data. It is also always possible for us to enter substance data you provide to us, or which we measure for you in our laboratories, into our programs for the calculations.

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