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pbrod(Industrial) 3 Oct 12 17:00

dear all,

we have a vacuum problem in a dimethylphtalate/water-methanol stripping column. after a

washing section the mixture with aprox 2% water/methanol is entering the stripping column at

about 130C.

a vacuum unit (Nash) should keep the column at 50mbara using 3barg steam. we have noreasons to believe that our top condensor (15C) water is not working well.

we equipped our three stage ejector system with manometers. when working well the pressures

are as following:

40mbara vacuum

120mbara between the first and second ejector. the outlet of the second ejector is to a condensoron 45C water (shell and tube)

500mbara before the last ejector. the last ejector is condensed on a 15C water (shell and tube).

we allmost always have a pulsating regime on the vacuum.

remarquable is that when we reduce the steam pressure from 3barg to 2barg is that the pressure

before our last ejector goes down to 340mbara.the drive steam is dry and the condensors of the ejector system is checked for fouling.

we also checked for flooding of the condensors but this was not the case.

do we need to look deeper into the ejector system?is this a normal behaviour of our last ejector?

in advance, many thanks for the help and tips!!

very best regards,

pieter

MatthewL(Chemical) 4 Oct 12 13:32

Pieter,

Did you check to make sure the outlet of the condensers have a long enough vacuum leg, and

that there is enough water available to seal the vacuum leg? We use a 40' (12 m) vacuum leg forour barometric, direct contact condensers on our Nash system. For the pulsation, look at your

condenser cooling water temperature, we notice vacuum fluctuations that correspond to

fluctuations in the cooling water temp.

Hope this helps,
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Matt

Matt

Quality, quantity, cost. Pick two.


dear Matt,

thanks for the response!

we did multiple tests with our vacuum leg. We now work with a seperate 20m leg straight down

from the intermediate and end condensor. so normally this could not be te problem.

I will log for some time the water temperature today. but we are running with a seperate coolingunit that sets the water temperature on 6C instead of 15C and the 45C water circuit

(intermediate condensor) is now running on 28C.

what was also noticed is that when we loose vacuum in our column 100mbara drop to 500mbara

(flashing in the feed line) the first 3 meters of the vacuum pipe (50mm) inlet to the ejectors is

getting hot.can this be explained?

best regards,

pieter

Compositepro(Chemical) 5 Oct 12 14:46

Have you checked your ejectors for wear or deposits? When the diffuser nozzle that the steamjet flows through gets too large you will get back-flow around the steam jet when thedifferential pressure gets too high.

MatthewL(Chemical) 5 Oct 12 15:54

Pieter,

The hot inlet is probably due to the higher pressure in the column. At 100 mbara, saturated

steam (the assumed condition in the column, since you are flashing the water) is 46 C, but at

500 mbara it is 81 C.

Regards,

Matt

Quality, quantity, cost. Pick two.

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we checked the ejectors for deposits and wear but could not find something (but we do not have

the original orifice diameters). also the condensors were checked for fouling.

Matt, the high temperature is at the inlet of the ejector system. between our top condensor outlet

there is about 20 meter pipe towards the ejectors. And it is at the ejector side of the pipe that wecan notice this temperature increase. After our top condensor we can't notice this temperature

increase.

i also measured the temperature drop on the intermediate condensor and this was 2C.

anyone experience with calculating the steam nozzles, venturi nozzles?

we will do a capacity test using orifices and restart the leak tests on the column.

chief(Marine/Ocean) 8 Oct 12 3:21

The ejector nozzles are usually of stainless steel construction and do not generally wear very

much. Check the throat body of the ejector, this may be of cast iron material and wears,

reducing the vacuum efficiency over time.

Offshore Engineering&Design


the throat body of the ejector is also stainless steel. we checked this and it looked OK. thanksfor the tip!

pierreick(Chemical) 8 Oct 12 8:01

Is your steam ejector jacketed ? Any risk for ice formation ?

Pierreick


last campaing we checked the convergent-divergent section and there was no risk for iceformation. now the ejector system is insulated.


a vacuum phenomena:- to give anyone an idea about the problem, in the attachemen there is a trend of our vacuum. thegreen line (1) is the vacuum. the scale is from 0mbara to 200mbara. the time axis is 40 hours.

the purple line is the temperature of the 15C cooling water circuit which cools the end

condensor and the top condensor of the stripping column. the red line is the level in the bottomof our stripping tower.

it is clear that the vacuum can be stable and low for some hours, then it goes back up and later
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stabelises at a higher level.

our top condensor is a alpha rozenblads lamella heat exhanger. its a counter current system, oval

tube, one pass.

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Have you considered a check valve on the final ejector stage. This is common on Steam turbine

plants, and a problem with low vacuum can be traced to a faulty check.



http://www.uniquesystems.com/pvs/brochures/Bulleti...

You may some pointers here,would appear you may have worn ejectors.


Longinthetooth(Chemical) 10 Oct 12 7:52

Echoing the comments on worn ejectors. You should check for cracks also. Even a tiny crack onthe tip of the nozzle can distort the jet and cause it to lose it's efficieny. I have seen this several

times.


tomorrow we will do a pressure test on the stripping column to search for leaks.

we will also do a second check of the ejector system, wear, steam quality,...

to chief: where is the check valve installed?

is it a possibility that the gas speed in the top condensor (of the stripping column) is to low tohave a good heat exchange?

rmw(Mechanical) 10 Oct 12 21:19

you need to check were the nozzle threads into the housing of the ejector. Steam can develop aleak past the threads and that adds steam to the load and load to the condensers.

rmw


The check valve is usually at the first stage suction manifold, this prevents 'leak back' and
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maintains the vacuum.


mjpetrag(Mechanical) 3 Nov 12 10:54

I've had a similar problem with a binary distillation column. As we increased reflux and gotmore of the light component in the overheads, the vacuum will fall off. When we cut the reflux

and stripped more of the heavy component out, the vacuum would drop back to set point.

Trend your all your flows (steam, feeds, etc) with the top pressure in the column. That may helpus determine where the source of the problem is.

Also, do you have parallel offline ejectors? If so try swinging on to them. Also, make sure that

if you have parallel ejectors that the steam, discharge & inlet process valves are blocked in tothe offline one. If these are opened, the discharge of the online jet can recirc back through the

offline jet and screw up the online jet.

A picture of your system with pressures and temperatures will also help us determine what is

happening.

If you are checking for a leak, block in all flows to the column and let the ejectors pull down

vacuum until it flatlines at the lowest pressure. Block in the ejectors and let the pressure start

building. If th e pressure starts coming up quicker than 4-5 mmHg/minute, you have an air leak.

-Mike

Question

How do Ejectors Work?

Answer

Operation of Ejectors is based upon Bernoullis Principle which

states: -

When the speed of a fluid increases its pressure decreases and vice

versa.

The principle is demonstrated by air moving over the top of a piece ofpaper is moving quicker

than the air underneath. Thus, the local pressure on the top surface of the paper is less than

on the underside. The resulting pressure imbalance causes the paper to rise.

First Diagram
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Shows a length of pipe which includes a valve. The pipe isarranged to discharge to atmosphere. At the point of discharge

there is a restriction (or a nozzle). The upstream side of the

pipe is connected to a pressure source. The valve is closed so

there is no flow. Upstream of the valve there is pressure

energy. The arrangement is very similar to that of a gardenhose connected to a domestic water tap.

Second DiagramWhen the valve is opened the fluid can pass through the pipe and discharged out of the end.

Because there is a nozzle at the discharge of the pipe, we can make the following observations:-

a) There is a pressure on the upstream side of the nozzle.b) A jet of fluid, moving faster than the fluid within the pipe,

emerges from the nozzle.

So, on the upstream side of the nozzle there is high pressure

and low velocity and at the nozzle discharge there is low

pressure and high velocity.

The nozzle has converted the pressure energy available upstream of the nozzle into kinetic (or

velocity) energy.Now, if we were able to see the surrounding air in the region of the nozzle discharge, we would

see that there would be eddy currents of air, circulating around the jet. In other words, the jet of

fluid emerging from the nozzle has imparted some of its kinetic energy onto the surroundingair.

Third DiagramIf we then placed a tube with open ends around the area of the

nozzle discharge, we would see that the eddy currents had

disappeared and that they had been replaced by a steady flowof air moving through the tube, in a direction from left to right,

as shown in the diagram.

Fourth Diagram

If we then blank off the upstream end of the tube and added aside inlet, we would see that the air would be sucked in through

the side inlet and discharged from the end of the tube. We now


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have a simple device that is capable of pumping the surrounding gas. This is a very basic formof an Ejector.

SummaryThe device we have created uses the available pressure energy in a fluid to do work on (pump)a secondary fluid.

Ejector DesignThis diagram shows the basic components of an Ejector used in the Oil and Gas industry.This Ejector was designed for use with gas. It has similarities with the basic Ejector

developed on the previous slide.

These are:-

1. There are three connections. One for the high pressure fluid, one for the gas sucked in(or entrained) and onefor the discharge.

2. The gas entrained comes in at the side.3. There is a nozzle for converting the pressure energy of the high pressure fluid into

kinetic energy.

The biggest difference between this and the previous slide is the venturi shape towards the

discharge end of the Ejector. This part is called the Diffuser.

The Diffuser is designed to firstly mix the two incoming streams. Then, when mixing is

complete, the diverging section slows the mixture down, thereby increasing its pressure. This isthe reverse of the process occurring in the nozzle. This feature enables the Ejector to discharge

at a pressure that is greater than that at the suction branch. Thus, the Ejector is capable of

compressing or boosting the pressure of the fluid entrained.

SummaryEjectors use a high pressure fluid to compress low pressure fluid to an intermediate pressure.

Other Terminology


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Ejectors are also often referred to as Jet Pumps or Eductors, but in gas production they arecommonly referred to as Gas Ejectors

and in oil production, Jet Pumps.

Operating Principle of Steam Jet Ejectors

Contact Us

An ejector has two inlets: one to admit the motive fluid, usually steam (inlet 1), and the other toadmit the gas/vapor mixture to be evacuated or pumped (inlet 2).
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Motive steam, at high pressure and low velocity, enters the inlet 1 and exits the steam nozzle at

design suction pressure and supersonic velocity, entraining the vapor to be evacuated into the

suction chamber through inlet 2. The nozzle throat diameter controls the amount of steam to passthrough the nozzle at a given pressure and temperature.

The entrained gas/vapor flow and the motive fluid (steam) flow mix while they move through the

converging section of the diffuser, increasing pressure and reducing velocity. The velocity of this

mixture is supersonic and the decreasing cross sectional area creates an overall increase inpressure and a decrease in velocity. The motive fluid slows down and the inlet gas stream picks

up speed and, at some point in the throat of the diffuser, their combined flow reaches the exact

speed of sound. A stationary, sonic-speed shock wave forms there and produces a sharp rise in

absolute pressure. The shock wave in the diffuser throat changes the velocity from supersonic tosub-sonic.

Then, in the diverging section of the diffuser, the velocity of the mixture is sub-sonic and the

increasing cross sectional area increases the pressure but further decreases the velocity.

The net result of these energy transformations is an increase of the absolute pressure of the

mixture on discharge to several times the pressure at which it entered the ejector inlet.

The choice of the ejector system will primarily depend on the final vacuum to be achieved. The

presence and, indeed, the amount of condensables in the gas stream will influence the choice ofsystem, as will the operating costs and capital available.

A Nash vacuum pump may, of course, be used as the final stage of any package, an option that is

often employed to reduce operation costs.

Air Ejectors

Pressurized motive fluids other than steam, such as ethylene glycol, air or nitrogen, follow the

same operating principles as steam ejectors.

This also applies to ejectors using air at atmospheric pressure as motive fluid, but these systems

are limited in compression ratio. The energy in motive air is developed with the help of a

vacuum pump, operating at lower pressures, pulling atmospheric air thru a diverging nozzle,


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increasing its velocity and dropping its pressure at the nozzle exit. This process, as in a steam

ejector system, entrains process gases thru the suction connection.

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