science

Larner–Johnson valve

Larner–Johnson valve is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Larner–Johnson valve rather than just read about it. In short: A Larner–Johnson valve is a mechanism used in dams and water pumping to control the flow of water through large pipes. The valve is suited to handling high velocity flow with minimal turbulence, even when partially open, and the actuating force can be provided by the water flow it is controlling.

Key takeaways

Larner–Johnson valve belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Larner–Johnson valve to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Larner–Johnson valve from memory before moving on to harder problems.

Reference excerpt

A Larner–Johnson valve is a mechanism used in dams and water pumping to control the flow of water through large pipes. The valve is suited to handling high velocity flow with minimal turbulence, even when partially open, and the actuating force can be provided by the water flow it is controlling. It was manufactured in the early 20th century by the Larner-Johnson Company in the US. The valves are still manufactured in the United Kingdom by Blackhall Engineering Ltd. These valves have been constructed in sizes up to 21 feet (6.4 m) diameter and controlling a hydraulic head of 1,000 feet (300 m). In 2009, Blackhall Engineering supplied four 60" bore Larner–Johnson valves to New York City Department of Environmental Protection for the Ashokan Reservoir in upstate New York. Each valve is capable of passing a flow rate of 19 cubic metres/second, which is the equivalent of 19 tons of water per second. The valves control the flow of water out of the Ashokan Reservoir into the Catskill Aqueduct down to New York City.

Operation

The valve is housed within a bulged section of the penstock pipe. The valve mechanism forms a cylindrical body within this pipe, with the water flowing around it. This downstream section of this valve body is conical and free to move axially. When it moves downstream, it seals against a conical surface at the outlet of the valve, closing off the flow. When closed, the valve is held shut by the supply water pressure, providing a good seal. The valve is hydraulically actuated, by the pressure of the flow that it is controlling. A servomechanism is used to generate the large forces needed for these huge valves, power being derived from the pressure of the water itself. Although the pressure within the pipe cannot be increased above that of the supply, it can be decreased. The pipe cross-section at the lower (downstream) part of the valve is reduced in section compared to that above the valve. By Bernoulli's principle, this increases the flow velocity of the water, thus reducing pressure. If water pressure is bled away through a drain valve, obviously that will reduce its pressure too. Internally the fixed valve body is constructed as a cylinder, with the moving part of the valve within this as a piston. The conical valve is part of this piston. The piston is double-acting and splits the internal valve body in two, with water pressure on both sides. The upper side of the piston is larger, the full diameter of the piston, and is supplied with water through a throttling valve; normally adjusted and then left in position. The lower side of the piston is an annular space surrounding the moving valve body and maintained at the lower (Bernoulli) water pressure. The tip of the conical valve contains a small pilot valve, linking the upper chamber of the valve to the downstream drain below the valve. This pilot valve is controlled mechanically from outside the valve chamber and is the main control over the valve position. When water pressure builds up in the upper chamber, through the throttle valve, this forces the valve downstream and closes it against the seat. As the area of the piston on this side is larger, the pressure easily outbalances the smaller area of the annular piston below. If the pilot valve is opened, water drains from the upper chamber and its pressure is reduced. The pilot valve is always larger in flowrate than the filling through the throttle valve. Pressure in the annular chamber now forces the piston upstream, lifting the valve body from the conical seat and opening the valve. At intermediate positions, pressure in the two chambers is balanced and the valve remains part-open. This depends on the piston size, the pressure reduction downstream and the flows through the two control valves. The pilot valve gives a proportional control over main valve flowrate, with the opening being controlled, slow and gradual. If the pilot valve is closed, pressure now builds up in the upper chamber, overwhelming the lower pressure and force in the downstream chamber. The valve is once again forced closed. The enormous forces involved in a valve of this type can give trouble for many types of valve. In the Larner–Johnson valve a great advantage is that these forces are always balanced and act only within the valve body, not on its actuators. This gives freedom from distortion and great reliability.

References

External links

Photo of a Larner-Johnson Valve

Illustrations

Worked examples

Example 1 — a first encounter with Larner–Johnson valve

Start with the simplest possible case. Write down what Larner–Johnson valve claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Larner–Johnson valve before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Larner–Johnson valve ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Larner–Johnson valve

In research: Larner–Johnson valve appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Larner–Johnson valve in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Larner–Johnson valve is common in secondary-school and first-year university syllabi. It links to neighbouring topics Power station technology, Servomechanisms, Valves, so understanding it makes those chapters shorter.
In everyday life: Look for Larner–Johnson valve outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Larner–Johnson valve in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Larner–Johnson valve means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Larner–Johnson valve out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Larner–Johnson valve in simple terms?

Why does Larner–Johnson valve matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Larner–Johnson valve?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Larner–Johnson valve.