A poppet is a metal disk with a seal positioned inside a check valve. It moves upward when a suction pumping system is turned on, to allow liquid to move through the valve. It drops down into a closed position when the pump is turned off, and helps keep the system “primed.”
To install two poppets in a valve, instead of one, is to provide extra insurance. If, for some reason, one of the poppets sticks or otherwise fails to work when the system is turned off, the second one serves as a backup.
An impact valve containing two poppets, or disks, is referred to as a "double-poppet shear valve" and functions differently than a check valve. This type of shear valve is typically paired with a device that can trip the primary poppet. The unit contains two poppets. One is located in the top of the shear valve above the shear point and is spring-loaded but held in an open position at all times. If the top is sheared from the valve body, the mechanism holding the spring-loaded poppet open fails and a strong spring closes the poppet away from the dispenser (south), preventing fuel from within the dispenser to spill out.
The second poppet is typically built into a gate or swing assembly. This gate is connected to a sealed shaft that is connected to an arm on the exterior of the valve body and closes itself aggressively via a strong spring. This poppet allows manual actuation of the gate by simply turning the arm connected to it. This gate should always swing down so that if the top were to be sheared, pressurized fuel from the system would only push against the gate and therefore keep it sealed against release. Setting up this valve involves turning the gate so it is in the open position and swinging an integrated retaining clip near the arm into place to hold the arm in the open position. This retainer is sometimes spring loaded in order to reduce the force required to trip the poppet. The retaining clip is typically connected to a ball-chain line which is then connected to a mechanical trip system located at the lowest point of the UDC or Secondary Containment sump. The trip mechanism is designed to react to liquid buildup in a cup located at a point lower than the bottom of the containment sump itself. Liquid buildup will raise the float - which actuates an arm and pulls the chain - thus pulling the retaining clip free from the spring-loaded arm that holds the gate-style poppet in the open position - resulting in an immediate closure of the poppet.