Reduction Of Damage In Blow Lines By Dr.-Ing. Dirk A. Lindenbeck, President, Stainless Valve Co.
Abstract
In pulp mills with common blow lines from a series of batch digesters to blow tanks often problems are experienced in respect to short life of blow valves, blown out gaskets, shaking of the equipment, short life of targets in the blow tanks, etc. The reason for these damages are leaking blow valves, kinetic energies in the moving pulp, deficiencies in the blow line design and improper programming of blow valves, diverter valves and isolation valves. The kinetic energies and flashing of cold liquor in the blow lines create shock waves, which are well above 10000 psi. Several suggestions are made, which are reducing the damages experienced in the blow line system.
1. Introduction
2. What Causes Shock Waves In Blow Lines
2.1. Flashing
2.2. Fast opening of blow valves
2.3. Re-directing of flow in a partially open ball valve
2.4. Temporary plugging of blow lines during a blow
2.5. Temporary plugging of blow lines at the beginning of a blow
2.6. Inertia of accelerated flow media
3. What Are The Consequences Of The Shock Waves In Blow Lines
4. What Can Be Done To Decrease The Effects Of Shock Waves
4.1. Steam Padding
4.2. Blow Line Design
4.3. Individual Blow Lines To The Blow Tanks
4.4. Expansion/Shock-Absorber
4.5. Blow Valves With Tight Shut-Off
4.6. Diverter valves
5. Conclusion
1. Introduction
In most of the pulp mills in North America batch digesters are connected to the blow tanks by common headers, where 2 to 10 digesters blow into. Common blow lines cost less to install than individual blow lines from each digester to the blow tanks. But for the operation common blow lines have a series of disadvantages.
In the following we will discuss the related problems and some measures to minimize the negative effects.
2. What Causes Shock Waves In Blow Lines
Figure 1 shows a typical situation of 3 digesters being connected to a common blow line to the blow tank.
2.1. When any of the digesters blow the hot pulp and black liquor comes into contact with remaining cold liquid from previous blows in the common header. This cold liquid flashes and thus causes shock waves in the blow line.
2.2. When any of the blow valves open and the opening is done very fast the pressure in the digester of over 100 psi expands suddenly into 15 psi environmental pressure. This sudden decompression causes forces onto the digester system, which can shake the digester and the piping. When the accelerated pulp and liquor hit an elbow, reaction forces cause shaking of the system.
2.3. When during the start of a blow, a ball valve is in the partially open position, the flow of material is re-directed twice as it passes through the valve. Again reaction forces are generated, which cause shaking of the system.
2.4. Some blow line systems are built using very short elbows. In such situations the flow of pulp and material may come to a stop, indicated by a 0 psi pressure measured in the blow tank, as opposed to 3 psi when normal flow of material enters the blow tank. When the pulp and liquor then suddenly start flowing again, considerable shock waves are experienced. I have witnessed saw dust, lying on a blow line, jumping up 2".
2.5. When a blow is supposed to start, especially of digesters which are furthest away from the blow tank, it is possible that the blow line is completely plugged up with pulp and liquor from a previous blow of another digester. That material does not move when the blow valve is opened. After some time and some heat transfer into the blow line that plugged up material will start to move suddenly and get accelerated by the pressure differential between the digester and the blow tank. Again the sudden acceleration of the material hitting elbows etc. causes shockwaves in the system and shaking of the system.
2.6. As an example we consider that the blow valve on the digester on the right, closest to the blow tank, opens and a blow starts. The cooked wood chips and the black liquor flow at the junction to the common header. Because of the shape of the piping the preferential flow will be towards the blow tank. However, there is no force that will prevent the pulp and liquor also to flow into the common header away from the blow tank towards the digester, which is furthest away from the blow tank, the one on the left. The pulp and liquor are accelerated by the pressure in the line and eventually come to a dead stop at the blow valve of the digester furthest away from the digester. The inertia of the mass of chips plus black liquor causes very high forces acting on the blow valve at the digester, which is furthest away from the blow tank, when the moving material comes to a sudden stop. Measurements of shock waves caused by this inertia have been found to be in excess of 10,000 psi. Also re-calculating the forces necessary to cause damage found on blow valves indicated that the forces are in the range of 30,000 psi.
3. What Are The Consequences Of The Shock Waves In Blow Lines
The first consequences seen when shock waves were heard on a blow system is leaking gaskets. Typically the gaskets between the blow lines and the blow valves are the ones, which show the leaks. Furthermore there may be gaskets in the blow valves, which start to leak. In more extreme situations, valve components may be deformed by the high kinetic forces. Cracks may develop in the blow line welds or welds in the valve bodies. In any case the damage may require shut down and replacement of components of the blow line system.
4. What Can Be Done To Decrease The Effects Of Shock Waves
There are some ways to reduce the occurrences and/or the effects of such shockwaves in the system.
4.1. Connecting a steam line to the common header with a 50 psi steam purge has the advantage that the blow line stays hot, and any liquor staying in the line will stay warm. This will minimize the flashing when the hot flow media from the digester hits the colder material in the blow line. This measure also has the advantage that the steam acts as a shock absorber when the flow media is accelerated in the common header and rushes towards the digester furthest away from the blow tank.
4.2. Designing the blow lines in such a way as to minimize cold liquor remaining in the line after a blow.
4.3. A more expensive solution is the design of the system in such a way as to having individual blow lines from each digester to the blow tank.
4.4. Installation of an expansion/shock-absorber on an elbow close to the blow valve on the digester furthest away from the blow tank. Such an expansion/shock-absorber may be advisable also on other elbows for other digester blow valves depending on the actual piping runs in the system. Picture 2 shows such an expansion/shock-absorber at the left digester. The effect of this expansion/shock-absorber is described as follows. When pulp and liquor are accelerated from the digester on the right towards the blow tank and also back in the common header towards the left digester, that material will go straight into the expansion/shock-absorber back up from there and prevent the flow media from rushing through the elbow and hitting the closed blow valve. The backed up material in the expansion/shock-absorber acts as a break and slows down the flow speed so far that the material hitting the left blow valve does not get hit with the high kinetic energy accumulated in the accelerated flow media. It is important to have a very substantial blind flange on the end of the expansion/shock-absorber. One customer put a ¾" thick blind flange on the expansion/shock-absorber and was astonished to find that blind flange badly dished. The blind flange on the expansion/shock-absorber should have at least a 2" thickness for a 10" pipe.
There is no need to clean out this expansion/shock-absorber; since the main effect is the creation of a slow down effect instead of a smooth elbow to the blow valve.
4.5. Another important aspect for the proper functioning of the blow system is to use the right non-leaking blow valves. Historically ball valves have been widely used for this application. However, as explained above, the twice re-directing of the flow through a partially-open ball valve, the exposed seats in the partially open position, metal seats, as well as the undersized design of ball valves for blow valve applications cause ball valves in this application not to perform as well as the O-port style of blow valve. When the first ball valve was replaced by an O-port valve (Figure 3), the customer had between two weeks and two months of life of the ball valve. Now, typically 3 to eight years is the time between refurbishments of O-port valves.
Leaking blow valves not only cause loss of chemicals, loss of energy, change of pulp properties, but they also leave cold liquor in the blow lines, which causes the above mentioned problems. Recently a customer pointed out findings in the blow system, which had partially ball valves and partially O-port valves installed with individual blow lines to the blow tank. The customer showed two targets, which had been in service for the same time, one from the blow line with the ball valve, the other from the blow line with the O-port valve. The target from the ball valve blow line showed considerably more corrosion wear than the other target. The interpretation, which the customer gave, was as follows: The metal seated ball valve leaks through, especially during the filling of the digester, when there is no force to seat the ball on the downstream side. The highly concentrated white liquor remains in the blow line. When the digester blows the first material to come out of the blow line hitting the target is the highly concentrated white liquor, hence the strong corrosion of the target. The O-port valve does not leak through independent of the pressure onto the valve from the digester side. When the O-port valve opens the target will see black liquor, which has a much lower corrosion effect than the white liquor.
Another advantage of the O-port valves is the fact that the flow is at any stage straight through the valve without redirecting. An O-port valve user pointed out that the O-port valves have "a quiet blow" in comparison with ball valves.
Finally, it is less expensive to refurbish an O-port valve than a ball valve.
Adding up all the advantages of the O-port valves, they cause less operating costs than the ball valves.
Figure 3 shows a cut-away of an O-port blow valve.
4.6. What has been mentioned above about the use of the expansion/shock-absorber is also valid for another application in the blow line system, i.e. at the diverter valves where a common blow line goes to two different blow tanks.
Often the piping system is laid out in such a way as to having one line with a diverter valve going straight and another line coming off the straight line in a y-piece turning to the right or the left, again with a diverter valve blocking the flow. When the branched-off line is blocked and the diverter valve in the straight line is open there is no problem to be expected because the flow simply goes straight. However, when the diverter valve in the straight line is closed and in the branched-off line is open, then the pulp and liquor will hit the closed valve in the straight line and only then turn off to the side and flow to the other blow tank.
You are at your home and have a water faucet open and water running. When you now suddenly close the ¼ turn faucet you will most likely hear a bang going through your water lines in the house. The inertia of the mass of flowing water coming to a sudden stop creates the "big bang". The inertia of the flowing pulp and liquor hitting the closed diverter valve causes the "big bang in the blow line system with all the negative consequences as described above. When this inertia hits a sufficiently dimensioned blind flange then the more sensitive valves are saved from major damage.
Picture 4 shows the situation often found in a digester/diverter valve system. Picture 5 shows the preferred piping solution, which minimizes the damage to the diverter valves. Another solution to this problem is shown in Figure 6 and 7, instead of installing two single diverter valves only one valve with one inlet and two outlets is installed, which allows to switch between the two blow tanks. The flow is directed without coming to a dead stop in a pipe.
5. Conclusion
While there is no way to eliminate shock waves in common header blow line systems there are some measures to be taken to minimize the negative effects of these shock waves on the blow system especially on the blow valves. The causes of shock waves and the effect of the measures are explained.
