Multidisciplinary equipment supplier WAM South Africa says more stringent safety and environmental laws have led to companies conforming to the use of its over/under (VCP) pressure relief valve.
These new laws include the South African government’s imposition of European standards on the South African pressure equipment sector in October last year.

Further, the new pressure equipment regulations (PERs), promulgated under the Occupational Health and Safety Act, of 1993, previously referred to as the vessels-under-pressure regulations, came into effect in October 2009.

Competent persons qualified under the South African Institute of Welding and approved by the South African Qualification and Certification Commit- tee to pressure-test pressure vessels had until April 1 to conform with the new PERs.

“Since the laws on pressure silos have become a matter of safety for personnel and the environment, customers have been resorting to our VCP pressure relief valves,” WAM South Africa GM Emilie Marchand says.

She explains that a silo becomes a pressure vessel when it is filled and subjected to overpressure. In such cases, the VCP valve will open and release excess pressure exceeding 1 bar to prevent the silo from exploding or being damaged.

However, when the silo is being discharged, underpressure can also occur. “In this case, the valve will open up and allow pressure from the environment to enter the silo to regulate the pressure,” Marchand adds.

The VCP valve consists of a cylindrical casing with a bottom flange that is connected to a spigot on the silo’s roof. A disc-shaped inner steel lid for nega- tive pressure operation is held in position by a central spring rod, while an outside steel ring for excess pressure is kept in position by three spring rods, gaskets and a weather-protection cover.

Marchand says helical springs keep the valve lids closed when the pressure value remains within the preset limits. The three spring rods on the outside of the valve keep the external ring-shaped lid firmly closed as long as the force generated by the pressure inside the silo does not exceed the spring force.

“Once the pressure exceeds the preset value, the lid is pushed up and the pressure can escape. The smaller lid covers the central circular opening of the external lid from below. It is held in the middle by a single spring rod and is pressed onto the external lid by the normal air pressure inside the silo,” she adds.

In the event of suction pressure, the spring is compressed and enables the lid to drop. The air entering the silo from outside ensures rapid pressure balance and pushes the central lid back up into the closed position.

However, Marchand says, spillage on construction and cement plants is problematic, owing to the lack of a regulating body to monitor the pressure with which tankers offload their product pneumatically into silos.

“Tankers often increase their off- load pressure to help [speed up] the process. “This results in major overpressurisation in the silos, which causes the VCP valves to constantly open and spillage to occur. “This is worrying, as these valves are only supposed to open in the event of an emergency,” she points out.

It is difficult for plant managers to regu- late the tankers, which are owned and managed by third-party companies.

To deal with this challenge, WAM South Africa has offered its VHS membrane pressure relief valve as a solution.

The VHS is based on the same con- cept as the VCP valve; however, it has an outlet that enables it to be connected to a pipe. “If the valve does open, owing to overpressure, the spillage can be channelled back into the silo. This has various environmental benefits, especially with the spillage of fly ash or cement,” Marchand says.

WAM South Africa’s primary focus has historically been on batching plants; however, she states that the company is turning its attention to other sectors, including the agriculture, food and beverage and glass sectors.

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