Thicker sheet gaskets are more likely to blow out. Therefore stacking will increase the likelihood from that perspective.

Thicker gaskets will creep more than thinner. Therefore the loss of bolt preload (initial bolt load before putting in service) is more likely, also increasing the likelihood of leakage or blowout.

Why were the gaskets stacked in the first place? Was it to overcome a poor flange face surface condition? What gasket stress (psi) are you using?

Stacking of gaskets has occured for two reasons mainly. The first being poor flange spacing so more gaskets were added to accomidate the opening. The other reason stacking of gaskets has occured is because maintenance forgot to take the old one off the flange prior to dropping a new one in the flange opening. As far as the rating goes for the garlock material. I am not sure what the rating is on the material.
What I am really looking for is some documentation as to why this shouldn't be a common practice.

Thanks

Mike