Abstract: A vessel pressure testing apparatus includes a safety retainer unit. An expanding plug unit is secured within a tubular outlet pipe of the vessel with a clamping bolt extending outwardly from an expanding rubber plug member. An attachment plate is secured abutting the outlet pipe with the clamping nut bearing on and locking the plate to the pipe and plug unit. A locking plate has a central opening larger than the external diameter of the pipe, and is passed over the pipe. A loop connector has one loop passed through an opening in the attachment plate and a second loop passed through a similar opening in the locking plate. The locking plate is located with the inner end spaced axially inwardly on the pipe relative to the outer end which is connected to the locking plate. Pressure is then applied to the vessel, and if the test plug fails for any reason, the plug unit tends to move from the pipe end.