With last week’s blog we looked into the pumping issues regarding subway systems such as New York’s. This week we will look more specifically into some of the design benefits, applications and technology that these pumps utilize to keep the subway system dry, as well as a brief look at other styles of pumps that can be used in similar situations on different scales.
Engineering behind the pumps
Mechanical engineers have been at work over the years designing and developing different types and styles of pumps to keep up with demands for pumps that are larger, more powerful and more efficient. The primary type of pump used in the subway systems are known as centrifugal pumps and they work by converting rotational kinetic energy to the hydro dynamic energy that is used to move the fluid from one place to another. Centrifugal pumps have a variety of advantages over other types of pumps, making them a logically reliable choice for the New York subway system. The largest advantage is that a centrifugal pump has no drive seals, meaning that leaks are not a factor. Also, there is no heat transfer from the pump’s motors because the chamber is separate from the motor, which also stops any liquid from leaking into the motor. Lastly, the pump is self-regulating due to magnetic coupling which separates if the load is too great so that it doesn’t get overloaded and damaged. For this same reason, however, if any ferrous or magnetic material goes through the pump, it can build up on the magnets and begin to restrict flow. Most centrifugal pumps are not self-priming, meaning that the pump can’t work properly without a liquid in it. If it fills with vapor or gas then it becomes incapable of producing the correct centrifugal forces needed to allow the fluid to flow and will ultimately cause damage and fail. To be sure that the pumps of New York’s subway are constantly running they must always remain submerged and maintain suction with the running water. Water enters the pipe through a hole located at the bottom of the pump and powers a shaft connected to a rotating impeller that accelerates the water. This rotational energy is created by the use of an engine or electric motor, so generators must be maintained along with the pumps. The water then experiences resistance from the pump casing which results in pressure building up and pushing the fluid out of the pump. Similar to a water wheel, the water spins the impeller which in turn accelerates it down a delivery pipe which drains it from the subway. The reverse operation of this type of pump is used to create energy and is considered a water turbine, which we discussed in a previous blog.
Other pump designs discussed
While the basic technology of pumps has changed very little in recent decades, they have grown larger and thus stronger. Mechanical engineers work with programs such as Computer Aided Design (CAD) and Computational Fluid Design (CFD) to design and test new pump technology. When it was noticed that the current subway pumps would fall short of the required load many engineering and manufacturing companies began shipping hundreds of pumps of many varieties, to help as reinforcement pumps. One of the differences between many of these pumps and the installed centrifugal pumps of the subway was that these were dry-prime models. This means that they use a compressor to blast air through a tube known as a venturi, named for the Italian physicist Giovanni Battista Venturi. The venturi effect creates vacuum which pulls air into a hose, similar to how a straw works. Another type of pump is known as a hydraulic submersible pump. These pumps are dropped into deep floods of water where normal machinery can’t reach which are normally below ground level, such as the basement of a building or storm drains in parking garages. An engine located away from the pump forces high pressure oil through a hose that travels down to the pump and spins an impeller which pushes the water into the discharge hose. Some estimates show that some of these portable pumps can manage about 16,500 gallons per minute, about four times faster than technology from just 30-40 years ago. With the subway’s pumps being near a century old it may be time to look at possible replacement pumps that can keep the subway running during the next super storm or hurricane.