One of the most misunderstood physical characteristics of pumps is the concept of head. This is related to stress, but what is it?
This article will uncover the mystery of the term "head" because it is related to the pump, so you no longer have to worry about what the head is, its relationship with pressure, or why it is important.
The definition of this concept is actually very simple, but it can be confusing when this concept is translated into an example involving a real pump. Imagine a pump with a pipe extending vertically upward from the discharge port.
Simply put: the pump head
is the maximum height that the pump can overcome gravity pumping. Intuitively, if the pump can produce more pressure, it can pump the water higher and produce a higher head. Also note that the higher the liquid in the tank, the higher the pump's ability to pump the water into the vertical discharge pipe due to the pressure head exerted by the liquid sucked into the tank.
A more useful head measurement is the difference between the liquid level in the suction tank and the head in the vertical discharge pipe. This number is called the "total head" that the pump can produce.
Increasing the level of the liquid sucked into the tank will result in an increase in the head, and decreasing the liquid level will result in a decrease in the head. Pump manufacturers and suppliers usually don't tell you how much head the pump can produce because they cannot predict the height of the liquid sucked into the tank. Instead, they report the total head of the pump, which is the difference in height between the liquid level in the suction tank and the height of the water column that the pump can reach. The total head has nothing to do with the liquid level in the suction tank.
Note that now that we have defined the total head, we can convert these useful relationships into equations:
Ht = Hd – Hs
Where Ht is the total head, Hd is the discharge head, and Hs is the suction head. Please also note that this equation applies regardless of whether the suction head is positive (the liquid level in the suction tank is higher than the pump) or negative (the liquid level in the suction tank is lower than the pump). In this case, the pump will still produce the same total head, but since the suction head is negative, according to our equation, the discharge head will be reduced by this amount.
The pump transports the liquid from the suction tank to the vertical pipeline, where the liquid rises until it cannot overcome gravity and stops rising. In this case, the flow rate of the pump is zero. The pump is working, but gravity stops the water in the vertical discharge pipe from rising and the net flow stops. This is called the "closed head" and it is the head that the pump can produce at zero flow.
To choose the pump you need, you need to know two things: the total head and the required flow. As you might expect, these two quantities are related. The maximum head (close head) is achieved when the flow velocity is zero. When the liquid flows along the pipeline from the suction tank to the pump, and then from the pump to the discharge pipe, increasing the flow rate will bring friction to the system. This friction reduces the total head that the pump can produce. In fact, as the flow rate increases, the friction increases, and the total head continues to decrease. The amount of water head lost due to friction is called "friction head" or "friction loss".
In a system with flow, the total head is the difference between the discharge head and the suction head plus the friction head. The sum will be less than the shut-off head. The relationship between the head and the flow rate is called the performance curve of the pump.