737s do not have fuel dump systems. A fuel dump system is a system which gets rid of any unneeded fuel if the aircraft needs to make an emergency landing while it still has a lot of fuel on board. This is useful because aircraft can only land safely at a certain weight or below. Getting rid of unneeded fuel helps to get to a safe weight for the plane to land. The first 737s were too small to need these systems. Adding a fuel dump system to the bigger 737s would make the plane much heavier. Instead, if the emergency is not very serious, 737s fly around in circles to get rid of fuel. If the emergency is serious, the 737 just needs to land at an unsafe weight. If it has to land at an unsafe weight, the plane needs to be looked at for damage. If there is no damage, the plane can be put back into service.