Influence of Overpressure in Pressure Vacuum Safety Valves on Emission Reduction and Explosion Risk Minimization of Atmospheric Storage Tanks

In today's economically competitive environment it is important to utilize a venting technology on storage tanks which provides the maximum degree of safety

of the protected item and personnel coupled with minimum product losses and explosion risk. On atmospheric vessels best practice in venting foresees the

installation of a correctly sized Pressure Vacuum Safety Valve (PVSV) with a low set pressure tolerance. A low set pressure tolerance is the capability of

the disk to remain seated when the inlet pressure approaches closely the set pressure. Furthermore, especially when the released medium is unrecoverable or

lost, it is recommendable to reach full opening with minimum overpressure, that is minimum pressure increment above the set pressure.

This paper compares different venting technologies on both seat tightness and product loss with the overpressure ranging from 10 %, as usually required for

safety relief valves (SRV) on pressurized tanks, to 100 %, which is the common technology for PVSV on the market. Product losses are calculated using new

equations derived from the German Standard VDI 3479 [1] and validated with an intense in situ testing at our TÜV certified test facility at the outskirts of

Braunschweig, Germany. The experimental testing confirms that gaseous emissions can be minimized during PVSV opening by using the 10% overpressure

technology, as derived by the theoretical equations and that seat tightness does not suffer from minimizing the overpressure requirements. Furthermore, by

minimizing the overpressure requirement higher set pressures can be imposed and the opening cycles can be shortened. In addition the product losses

during a complete opening and closing single cycle were investigated in the same range of overpressures and here again the 10 % technology presented the best

results, e. g. minimum product loss and shorten opening cycle.

For what regards the set pressure tolerance API 2000 5th Edition [2] suggests that leakage tests should be done at an inlet pressure equal to 75% of the (gauge) set pressure. In addition, it is stated there that weight loaded vents have a 0% blow down. Both statements are not correct and the blow down statement was corrected in the API 2000 6th Edition [3] and preserved in the 7th Edition [4]. Nevertheless, there are today a lot of operating venting systems designed using API 2000 5th Edition [2] presenting high product losses in potentially explosive atmospheres. This paper will pinchpoint also some of these systems and discuss engineering solutions to improve them by minimizing the risk of explosion and reduce the excessive product losses.

Bibliography:

[1] VDI 3479 Standard: Emission reduction Marketing installation tank farms (Emissionsminderung Raffinerieferne Mineralöltanklager), 2002.

[2] API 2000 5th Edition: Venting Atmospheric and Low-pressure Storage Tanks, 1998.

[3] API 2000 6th Edition: Venting Atmospheric and Low-pressure Storage Tanks, 2009.

[3] API 2000 7th Edition: Venting Atmospheric and Low-pressure Storage Tanks, 2014.