Demystifying Maximum Continuous Speed (MCS or MCOS)

A few months ago, while I was still discussing Terms and Definitions from Chapter 0, I explained the term Maximum Continuous Speed (MCS) or Maximum Continuous Operating Speed (MCOS).

Well, I just realized I never said what MCS is in terms of its numerical value or equation to calculate it.

When I started doing research to find its origin and source, I found out that this term is referenced daily by engineers and technicians in the rotating equipment industry to calculate balance tolerances, but its origin is quite elusive and its value is never mentioned in API RP 687!

You will find scientific publications, marketing material from rotating service companies, training material, etc. saying “API defines MCS as…”, but when you look for the actual reference, where is it???

One reason for its evasiveness is that original equipment manufacturers are the ones that should define what the MCS is for each piece of equipment.

By definition the MCS is this: Highest rotational speed at which a machine, as-built and tested, is capable of continuous operation.

Therefore, its true value must be calculated based on the design and operating conditions of each machine.

Why is it important to know the actual MCS?

The simple answer: for safety and accuracy of work.

At the shop where I work, we have a policy that our customers must formally confirm the MCS of their rotors so we can:

• Calculate the appropriate balance tolerances

• Avoid over speeding a rotor or an over-speed trip device during a repair, spin test, or high-speed balance

Before we get into what the MCS actually is, there are other speeds that are important to establish.

The many speeds of a Steam Turbine

From API 612 Steam Turbines in Special Purpose Applications, we can learn a list of speed definitions:

Slow Roll Speed

This speed is used to warm up or cool down a machine, so its rotor does not bow. Machines need to cool down calmly. For the same reason, our moms may have warned us not to open the freezer after we’d been playing soccer in the 102ºF summer heat.

By definition, this speed is less than 5% of the normal operating speed (or rated operating speed).

Minimum Critical Speed & Maximum Critical Speed

These terms are mentioned in API 612 in an awesome figure called Figure 4 – Mechanical Drive Turbine Ramp Speeds, but they are not given an actual definition.

These terms essentially define the edges for the range of the first critical speed.

A critical speed being the shaft speed in which a rotor-bearing system is in a state of resonance.

When things are in a state of resonance, there are usually high vibrations; and if these vibrations persist for an extended period, damages or failures can occur.

Minimum Operating Speed

This term is also not defined, but we can infer that it is the bottom of the range of speeds at which the equipment is designed to deliver its operating conditions.

Rated Operating Speed (RATED)

This is also known as 100% Speed. It is the highest rotational speed required to meet any of the design operating conditions.

Maximum Continuous Speed (MCS)

This is the famous MCS, and it is not explicitly defined as numerical calculation.

The definition states that this is the speed at which a turbine is capable of continuous operation the way it was built and tested.

Maximum Allowable Speed

This term is defined as the highest speed a manufacturer will allow the continuous operation of a machine.

In 22 years, I have never seen this value used for practical purposes in the context of a repair.

Overspeed Trip Speed (OST)

This is not formally defined in the definitions section, but from Figure 4 and equipment data sheets we know this speed is typically 116% of the rated speed or 110% of the MCS.

Maximum Temporary Overshoot Speed

This speed appears in Figure 4, but it is not defined in the document as a percentage value, and it does not have a calculation reference.

But API does say that if this speed is reached, a rotor will require immediate maintenance.

So, we must assume it is a design value that design engineers must use to drive their design calculations, but is not necessarily published because it does not have practical applications.

To summarize the important speeds, I have drawn them in the style of a speedometer below.

So, what is the MCS?

Like I mentioned before, there are references out in the world that site API defines MCS as 105% of Rated Speed.

But no API document explicitly or directly presents this as a formula that looks like this:

MCS = 1.05 x Rated Speed

API 612- Steam Turbine in Special Purpose Applications, says:

The closest we get is a statement in API 612 that says: The turbine trip speed shall be 110% of the maximum continuous speed (116% of the rated speed).

(Link: https://www.apiwebstore.org/standards/612)

From here we can derive:

Trip Speed = 1.10 x MCS

Trip Speed = 1.16 x Rated Speed

1.10 x MCS = 1.16 x Rated Speed

MCS = 1.16/1.10 x Rated Speed = 1.05 x Rated Speed

voila! behold the calculated value!

API 617 – Centrifugal Compressors and Expanders says:

API 617 does say: The maximum continuous speed shall be at least 105% of the "as-tested" rated speed.

(Link: https://www.apiwebstore.org/standards/617)

Not necessarily a firm formula, but we know that “At least” it should be a percentage of the “as tested” rated speed.

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And this is all we have. We’ve unlocked the mysterious origin of the elusive MCS!

Many engineers and references will use and cite the MCS of a machine and accept that it is commonly calculated as 105% of the Normal or Rated Operating speed.