What's the maximum size of orifice for minimum flow line?

[CaracasEC]

Hi All,
What could be the maximum size of orifice for minimum flow lines? Because control valve could save energy once minimum flow lines are big enough? Thanks for any comments in advance..

 

[katmar]

The range of orifice beta ratios (i.e. orifice diameter over pipe ID) usually given are to ensure accuracy in measurement and control. In a minimum flow line you would probably be OK with an accuracy of 10-15% so I would not be too fussed about some arbitrary rule about maximum size.

The classic paper in this regard is Hooper, Chem Eng, Nov 7, 1988, pgs 89-92 and he does not specify any limits for his correlations.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[CaracasEC]

Hi Katmar,
Thanks for the reply. But what if the size is large enough and the energy savings would be great if converted from orifice to control valve?

 

[pennpiper]

"But what if" you can "what if" things till the cows come home and still not get anywhere.

Put a simple manual globe valve in the minimum flow bypass line.

You will have the ability to "play" with the flow till you get it right. It is not fixed and you did not endure the cost of the full Control valve and all of the control loop hardware.

 

[JJPellin]

You did not provide enough information about the service. If the flow from the pump into the process is highly variable, a control valve might be justified. If the flow varies a lot, then you could be forced to size the minimum flow orifice to protect the pump with the lowest possible outgoing flow. That means that during times of higher outgoing flow, you are spilling back much more than necessary to protect the pump. That is energy wasted. Perform the energy calculations to determine the potential savings. Take the plot of outgoing flow over time and compare that to the minimum flow requirements for the pump. At each point, compare the horsepower demand of the pump with a fixed minimum flow versus a controlled minimum flow.

You could install a self-contained and self-powered automatic spill-back valve. The most common manufacturer we use has a brand name of Yarway. This will close off the minimum flow line when the outgoing flow is above the required minimum and open it only when the outgoing flow is below the required minimum.

If, on the other hand, your outgoing flow is constant, then there is little or no value in an automated valve. An orifice or a manual globe valve would be more than adequate.


Johnny Pellin

 

[BigInch]

Usually the question goes the opposite way, what is the smallest orifice. That's where the velocity gets high and you start flashing with cavitation and too much noise.

Don't look to save money by replacing an orifice with a control valve. Look to replace an orifice with a control valve to cut minimuze the noise and flashing.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[katmar]

If it is a large pump and the energy consumed by the spill back is significant then either use a self actuating valve as recommended by JJPellin or use an on-off ball or butterfly valve in combination (series) with the orifice. I cannot see an externally powered and controlled modulating valve ever being justified, unless this is a monster application. And then you shouldn't be asking us.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[BigInch]

Large pumps that have to develop a high static head to 2000 m or more, but can't even open the discharge check valve when warming up at low speed while waiting for suction pressure to build sufficiently from the upstream pipeline section to allow the pump to speed up and develop more torque to get the pipeline flow started will often need a control valve in the recirculation line. In fact a control valve with an orifice combined to split the pressure drop, minimize flashing and noise.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[CaracasEC]

Hi All, Thanks for the informative feedbacks,
information related to these pumps is listed below:

Item Rated Capacity Motor P Est. Recycle Power
G-0405A/B 8.9 + 3.3 15 4.1
G-0407A/B 8.2 + 3.3 45 12.9
G-0502A/B 17.7 + 8.6 75 24.5
G-0503A/B 35.2 + 17.2 22 7.2
G-0509A/B 20.7 + 7.8 22 6.0
G-0903A/B 37.8 + 22.1 45 16.6
G-0405 Wash Gasoline Pump, G-0407 Gas Oil Stripper Bottoms Pump, G-0502 Charge Gas Comp 3rd stage suction drum pump, G-0503 Charge Gas Comp 3rd stage discharge drum pump, G-0509 Medium Gasoline Pump, G-0903 PGH Depentanizer Reflux Pump. As you can see, estimated recycle power is an energy savings. This pump were purchased and to be installed according to API 610 Standard. On our Boiler Feed Pump application, we are using ARV (Automatic Recirculation Valve) which will automatically switch to main line once minimum flow is reached. These pump are all inside the process area of an Ethylene Cracker Plant which we are ongoing construction. Target commissioning of the plant will be on 2013. What i think is that 4kw above of recycle power should be change to control valve, which will shut-off at predetermined flow.

 

[BigInch]

ARVs work when the pump discharge flows go low, forcing the pump to back up towards shutoff head, the higher discharge pressure then opening the ARV diverting to the recycle line. They will not work so nicely, if you reduce pump suction pressure.

Assuming the pumps are constant speed, the energy consumed at full flow, or half flow, or any flow will be Power = Qt*[ρ]*g*h/eff. Where Qt is the total flow to downstream, Qd if any, plus flow to recycle, Qr. H is the head shown on the pump's curve at the same total flow, Qt. Whether the flow is going to recycle, via an orifice, or via a control valve, will make no difference to the energy consumption of these pumps. It is still P = Qt*[ρ]*g*h/eff. Because Qt remains constant no matter how much is going to recycle or how much is going downstream, but if you reduce Qt, no matter how you do it, by reducing Qr or Qd, pump Power will drop accordingly.

What you don't want to do is to have the recycle lines open when you are trying to move all of the design flowrate downstream, assuming your pumps are sized with the correct head for your design flow at BEP. You are the one wasting energy in that case, because you are not using that high pressure recycled fluid in your process, but the pump has no choice, it must use whatever Power happens to equal = Qt*[ρ]*g*h/eff.

There is no more energy savings possible here than what you would get if you simply reduce the total flowrate through the plant. To save energy, perhaps by considering VFDs, the next step would be to see how much you could save. Then you would see that they would cost you 5% more at full flow and you would save no energy running at design capacity. Then you would also see that you have to operate at reduced capacity for the VFDs to have any chance of saving money. Then you see, that if you reduce flow, you also reduce INCOME from the plant and that there is no future in that VFD logic. Its pretty much like saying, "Humm... If I shut this plant down, I will be able to save a lot of energy."

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[CaracasEC]

The plant is design on a 50% turndown capacity. Based on the formula given, Qt and h is variable on pump operation. That is once the head (h)increased which will happen at low flow, Qt will decrease and vice versa based on any centrifugal pump performance curve. Would this mean that at constant speed, energy savings wether using an orifice or an automatic control valve is not a big issue? thanks you very much for the informative comments.

 

[BigInch]

Without VFDs it is no issue at all. You will get your energy savings simply by the lower flowrate through the plant alone. Whether you will get more energy savings, if you use VFDs, is entirely another question. Over the last few years, I have studied the application of VFDs to new plant and facility designs and have found out that generally speaking,

With VFDs, pump energy essentially increases to 5% more than what you would use with simple constant flow at the optimum BEP speed design flow.

At around 50% flow, your energy consumption is obviously roughly 1/2, based on 1/2 Q alone, Power = q[ρ]h/e, neglecting some efficiency decrease. You have a potential to save money, if you can also cut H, but with VFDs you must be able to cut H by the drop in speed squared to move any flow. So, that's 1/4 H at design speed, a drop to 0.5 squared. Lots of plants can't do that.

So, generally speaking again, you can't drop to 50% with VFDs in many applications. If you do, you don't save all that much over constant speed operation, because you've already saved half anyway simply by cutting plant output in half, so there's only a max you will save around 25% at best, not half.

Here's the killer. Yes, if you operated the plant at half flow you might could save money with VFDs, but do you want to operate at half flow? NO! You've already cut your profit in half. There is no business reason to do it, so if you do, you will only run at half flow for the smallest amount of time possible, or your boss will kill you. Now let's push up toward higher flowrates again.

As you get to 75% to 85% operation, there is some potential to save money with VFDs, you're in the good range for that, but if you move to operation at 80 to 90% design for enough time where it makes it attractive to use VFDs, I've found that you should really review your design flow criteria and most likely lower that to the point where you can go back to running the plant continuously at a full speed operation again, just at 90% of the design flowrate that you thought you needed before. When applying VFDs to new plant design, if it looks attractive, it could easily turn out that you probably made a bad choice of original design flowrate and you should reduce that by 10 percent or so and run at constant speed. If you selected your design flowrate conservatively to have some easy future expansion capacity, then, if you were so optimistic then, you shouldn't now be looking at running at 75% or less design flowrates where VFDs make sense. VFDs have some potential to save in closed loop systems, where you may never want to operate at maximum flow for a very long time, otherwise ... well .... look elsewhere for energy savings. There ain't any there.





We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[CaracasEC]

To summarize, whether it is an orifice or a control valve energy savings is not a big issue on a pump driven by a constant speed motor. For a pump driven by variable speed motor (VFD's), energy savings viability will be 85% down.

 

[pumpkind]

As a rule of thumb (also used by few reputable petrochem companies ) , up to 15 kW installed power it is considered more efficient to have a permanent recirculation line fitted with an orifice. ( in which case the recirculation constant flow needs to be added to the required process duty )

 

[CaracasEC]

Thank you very much pumpkind for that rule of thumb practice. This is a new information to me...