| Term | Definition |
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| Make-Up | The water added to boiler feed to compensate for that lost through exhaust, blowdown, leakage, etc.
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| Manhole | The opening in a pressure vessel of sufficient size to permit a man to enter.
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| Manifold | A pipe or header for collection of a fluid from, or the distribution of a fluid to a number of pipes or tubes.
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| Manual Gas Shutoff Valve | A manually operated valve in a gas line for the purpose of completely turning on or shutting off the gas supply.
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| Manufactured Gas | Fuel gas manufactured from coal, oil, etc., as differentiated from natural gas.
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| Maximum Allowable Working Pressure | The maximum gauge pressure permissible in a completed boiler. The MAWP of the completed boiler shall be less than or equal to the lowest design pressure determined for any of its parts. This pressure is based upon either proof tests or calculations for every pressure part of the boiler using nominal thickness exclusive of allowances for corrosion and thickness required for loadings other than pressure. It is the basis for the pressure setting of the pressure relieving devices protecting the boiler.
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| Maximum Continuous Load | The maximum load which can be maintained for a specified period.
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| Maximum Instantaneous Demand | The sudden load demand on a boiler beyond which an unbalanced condition may be established in the boiler's internal flow pattern and/or surface release conditions.
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| Mechanical Atomizing Oil Burner | A burner which uses the pressure of the oil for atomization.
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| Mechanical Draft | The negative pressure created by mechanical means.
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| Micron | One millionth of a meter, or 0.000039 in. or 1/25400 in. The diameter of dust particles is often expressed in microns.
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| Miniature Boiler | Fire pressure vessels which do not exceed the following limits:
16 in. inside diameter of shell; 42 in., overall length to outside of heads at center; 20 sq ft water heating surface; or 100 psi maximum allowable working pressure.
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| MMBtu | Millions of Btus (British Thermal Units).
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| Moisture | Water in the liquid or vapor phase.
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| Moisture in Steam | Particles of water carried in steam, expressed as the percentage by weight.
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| Moisture Loss | The boiler flue gas loss representing the difference in the heat content of the moisture in the exit gases and that at the temperature of the ambient air.
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| Multifuel Burner | A burner by means of which more than one fuel can be burned.
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| Multiport Burner | A burner having a number of nozzles from which fuel and air are discharged.
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