Pump Minimum Flow - mechanical protection 5

[tkdhwjd]

Folks,

I understand that a percentage of the process fluid will recirculate at the eye of impeller and the percentage increases as flow drops. Heat build up due to this recirculation can potentially cause flashing, and hence cavitation as fluid velocity turns into pressure.

I also heard about "mechanical protection" of a pump at low flow. Can anyone please explain why the pump is mechanically unstable when the flow drops below the minimum continuous flow rate?

Perhaps the magnitude of flow reversed (shift in momentum) is so high that the impeller gets exposed to significantly unbalanced force?

 

[25362]

From past notes, the solubility of air in liquid hydrocarbons, is generally given by the Ostwald coefficient as by ASTM D2779.

It is estimated in two steps starting from a reference value. For hydrocarbons at 273K, this reference value is C_o = 0.095, namely 9.5% by volume.

As a first step, a temperature correction is obtained by:

C_T = 0.3 [×] e^{[0.639(700-T)/T [×] ln (3.333 [×] C_o)]}​

where T is the temperature [K]

Then, a fluid density correction is carried out:

C_d = 7.7 [×] C_T [×] (980 - [ρ])​

where [ρ] is the density [kg/m³]

I hope I'm not mistaken.

 

[BigInch]

25362,

Much thx (& *) for your post.

More info about "pumping air", quoted from mcnallyinstitude.com

A centrifugal pump can handle 0.5% air by volume. At 6.0% air, the results can be disastrous. Air gets into a piping system several ways that include:

Through the pump stuffing box. This problem occurs in any packed pump that lifts liquid or pumps from a condenser, evaporator or any piece of equipment that runs in vacuum.
Some pumps are equipped with a repeller that will lower the pressure in the stuffing box
Through valves above the water line.
Through leaking flanges.
Any vortexing fluid.
A pump discharge bypass line that has been installed too close to the pump suction.
The suction inlet pipe is out of fluid. This can occur when the tank level gets too low or there is a false reading on the gauge because the float is stuck on a corroded rod.
Both vaporization and air ingestion have a similar affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher-pressure side of the impeller. When the bubbles collapse as a result of air ingestion, they do very little damage to the impeller and casing walls. The main effect of air ingestison is loss of capacity.

Although air ingestion and vaporization both create bubbles they have separate solutions. The obvious solution for air ingestion is to stop air from coming into the system by correcting the above problems. Fortunately air ingestion is not as severe as vaporization.

 

[BigInch]

a little more,

10% gas in a liquid reduces the liquid's bulk modulus to appx 1/100 th of the original liquid's bulk modulus, thus volumetric changes with pressure are greatly increased. That would have a severe impact on pump efficiency.

 

[010874]

BigInch,

I would like to know the minimum pipe length (distance) reqd in the pump suction which can ensure heat dissipation, thereby ensuring no substantial heat rise, due to minimum flow recycle. I cannot put the recycled liquid back to the tank. I understand that there is some formula to estimate this minimum length reqd at the pump suction & am searching for it. But not being able to get the answer.

Pls anyone can help me out.

Anyways thanks for the reply.

010874

 

[BigInch]

Search for "Heat loss through a pipewall." "Cool", but I think my method will work better, if you don't have forced cooling on the pipe, because the heat disipated to atmosphere or ground probably won't be fast enough to avoid heating up the recycled fluid anyway. You could try. Its much easier just to consider a reservoir of fluid in the recycle line itself rather than including the OTHC calcs, which don't help a whole lot or depend on which way the wind blows and how cold it is that day.

It ain't easy to dig it up. Do it right the first time.

 

[25362]

Erratic, and evidently less predictable, NPSH behaviour of pumps moving gasoline and similar products is not necessarily due to the presence of light constituents; it is considered more likely the dissolved (not just entrained) air would be the culprit when evolved. Hydrocarbons having low vapor pressures are the most likely to become aerated when exposed to the atmosphere.

Tests carried out on lubes, diesel fuels, aircraft fuels and water, showed -with no exception- that the half-life for evolution was always shorter than that for solution, meaning gas evolution is quicker than redissolution. Air-binding effects as explained by Big-Inch may be the result.

If a bulk-station pump was selected using NPSHR on a water basis, its performance can be ruined since the hydrocarbon fluid may absorb great quantities of air when exposed to the atmosphere, and the NPSHR may need a factor of 2.0, or more, of water-NPSHR.

Interestingly enough, when pumping from fractionating towers (ie, air-free) pumps may qualify for about 0.9 of the water-NPSHR.

 

[25362]

On the other hand, it is apparent that on centrifugal pumps taking water from a cooling tower, the suction line is shortest to avoid small air bubbles from agglomerating and coalescing into large bubbles thus affecting the pump's performance. Any comment ?