Posts Tagged ‘valves’

The Principle of Continuity – What Goes In Must Come Out

Monday, April 2nd, 2018

    Last time we learned how cavitation degrades a centrifugal pump’s performance by restricting and reducing the water flow at the pump’s inlet where these destructive bubbles are formed.   Today we’ll see that despite the fact these cavitation bubbles return to a liquid water state further along in the pump’s high pressure section when they implode, the water flow within the pump remains the same.   This is true because of the engineering principle of continuity, which holds that the water flow rate within a pump or any other closed system remains the same throughout that system.   What goes in must come out.

    Continuity has to do with the rate of water flowing through pipes, valves, and pumps within a plumbing system.   As water flows through a centrifugal pump, its flow rate is measured as the volume of water that moves past a certain point in the pump per unit of time.   Suppose for example that during one minute of elapsed time the volume of water flowing through at the inlet point is found to be five gallons.   This then becomes the system’s flow rate of 5 gallons per minute.   We’ll call this flow rate Q1.

 The Priniciple of Continuity – What Goes In Must Come Out

The Principle of Continuity – What Goes In Must Come Out


    The principle of continuity states that this flow rate Q1 must remain the same throughout the pump.   If this were not true, any observed difference in water volume would mean water is somehow either lost or created between the pump’s inlet and discharge.   This is an impossibility if the pump is an intact enclosed system, absent any other inlet points or leaks.   So according to the principle of continuity, Q1 must equal Q2, the flow rate at pump discharge.

    When cavitation occurs at the pump’s inlet, Q1, these steam bubbles restrict water flowing into the pump.   Although these bubbles will later implode and return to a liquid water state further along the pump system, this change will not affect the flow rate of the water within the pump.   The flow rate established at intake will remain the same at pump discharge, Q2.

    Next time we’ll see how cavitation in centrifugal pumps can be prevented.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog




The Make-up Valve in the Power Plant Steam to Water Cycle

Monday, October 28th, 2013

      Last time we learned how the condenser recycles steam from the turbine exhaust by condensing it back into water for its reuse within the power plant steam-water cycle.   This water is known as condensate, and after leaving the boiler feed pump at high pressure, it’s known as boiler feed water.   Today we’ll introduce a special valve into the system, whose job it is to perform the important function of compensating for lost water.   It’s known as the make-up valve.

Power Plant Engineering Expert Witness

      The illustration shows the flow of steam and water within the cycle.    Tracing the path of orange arrows will reveal it as a closed system.

      Under ideal operating conditions recycled condensate from the condenser would provide enough water to keep the boiler indefinitely supplied.   In reality water and steam leaks are a chronic problem within power plants, even when well maintained.   Leaks typically occur due to worn parts on equipment, a condition which is commonly present due to the demanding operating conditions they must endure.   First, there is the strain of continuous operation, then there are the high temperatures, typically greater than 1000°F, and high pressures that pipes, valves, pumps, and the boiler itself must endure.   We’re talking about pressure higher than 2000 psi, that is, pounds per square inch.   As a result, water levels within the boiler must periodically be replenished.

      While tracing the arrows through the diagram, you would have come across the new make-up valve under discussion.   It’s located on the pipe leading from the power plant’s water treatment system to the boiler feed pump.   It’s normally kept closed, except under two circumstances, when the boiler is initially filled at startup, or when water replenishment needs to take place.

      Due to water loss and difficult operating conditions, maintenance within the water-to-steam system of a power plant is a never ending task.   There are miles of pipe connected to hundreds of pieces of equipment, all of which are distributed through a huge power plant structure.   So the reality is that power plants operate with a continuous eye on leakage.

      To contend with the leaks, human intervention is often required in the way of a boiler operator.   Their job is to manually open the make-up valve to admit a fresh supply of water from the treatment plant to the boiler via the boiler feed pump.   Once the system’s water requirements are replenished, the valve is once again closed.

      Next time we’ll continue this series by discussing how the condenser enables the steam turbine to run more efficiently by creating a vacuum at the turbine’s exhaust.