A Discussion on Yankee Safety Issues

1. A Discussion on Yankee Safety Issues By: D K Singhal

2. Disclaimer This presentation is supplied as a self learning presentation, and is subjected to the condition that the author accepts no liability from the use of the same or from the techniques or information provided within the same. The information provided herewith is supplied exclusively for mild steel fabricated yankee cylinders, and expected to be beneficial to the users, however in case of any problem, may it be operational, mechanical, breakdown or any other type, the author cannot be held responsible for the same. Users are welcome to consult the author in case any problem is observed. It is strongly recommended to consult your machinery supplier, yankee supplier, mechanical engineer, and process engineer etc., before making any changes to the existing system.

3. Issues… • To protect yankee from unwanted maintenance, e.g. grinding etc. • To ensure proper safety of two “M”s Man and Machine.

4. Common Reasons for failure… • Increased steam pressure inside yankee. • Increased condensate level inside yankee. • Increased touch roll loading. • Increased moisture with incoming paper web.

5. Pressure inside yankee • Pressure is normally controlled using a manual globe valve or alternatively with a pneumatically controlled valve set at desired setpoint. • Manual control has its own limitations, while pneumatic control may allow more steam to enter yankee cylinder in case of a malfunction.

6. Increased Condensate Level… • Increased condensate level can be due to following reasons- • 1. Siphon pipes leakage or broken. • 2. Steam trap failure, strainer chocking etc. • 3. Rotary joint sealing or guide rings broken.

7. Increased Condensate Level… • In most of the cases, an increase in condensate level remains unnoticed. This results undesirable load due to both increased weight of yankee as well as increased pressure inside yankee to compensate for reduced heat transfer area due to condensate.

8. Increased Touch Roll Loading • Sometimes, in case of pneumatic or hydraulic system failure (as the case may be), the load on touch roll may increase beyond permissible limits.

9. Increased Moisture • This is also possible that due to sudden change in vacuum, or some other process variables, the paper web moisture content increases suddenly resulting in undesirable stress on yankee.

10. STEAM PRESSURE CONTROL

11. Steam Pressure Control • It is strongly recommended to check all upstream side valves, pressure gauges etc. periodically to minimize the probability of failure of the same. Pressure gauges must be calibrated at regular intervals. • One mill reportedly uses two pressure gauges, one for normal use and another (normally valve shut off) for cross checking in case the reading of the first is doubtful.

12. Steam Pressure Control • In addition, a pressure switch should be installed at steam inlet line after final control valve, but before rotary joint. • This switch should be connected to an alarm to activate if the pressure increases beyond limit. Any increase in steam pressure to MG will therefore help the operators to do corrective action.

13. Steam Pressure Control • A safety valve should also be installed at the same line as above, set to bleed off some amount of steam from yankee in case steam pressure increases. Since, this is very rarely to operate, periodic checking and calibration of the same MUST be done.

14. INCREASED CONDENSATE LEVELS

15. Increased Condensate Level • In case siphon pipe or trey is broken or cracked, it becomes impossible for the condensate to reach to rotary joint. As a result, condensate level increases. • These should be checked periodically, at least once a year to ensure these are properly welded/bolted.

16. Increased Condensate Level • In case of plugged strainers, or malfunctioning of steam traps, condensate cannot be removed from yankee, and hence water level increases. Such situation occurs in case of air locking, steam locking, mechanical failure of trap or scaling deposition in steam trap and strainers.

17. Increased Condensate Level • Rotary joint carbon rings (seal rings and guide rings) if broken, may allow a part of steam from inlet side to escape from the condensate side without entering yankee at all. • Such a situation will result in increased pressure at condensate line, and hence condensate will not be removed from yankee.

18. INCREASED TOUCH ROLL LOADING

19. Increased Touch Roll Loading • Malfunctioning of pressure regulating valve in hydraulic or pneumatic system may result in sudden increase in touch roll loading. • In one study, it has been observed that out of total forces acting on a yankee, the touch roll loading accounts for nearly 24% of load.

20. Increased Touch Roll Loading • Carefully designed hydraulic or pneumatic systems with provision to control the pressure beyond a certain value are a must. • It has been observed that presence of moisture in pneumatic system results in premature failure of regulators. Air should be free from moisture. • In case of hydraulic system, periodical cleaning of oil filter should be done to avoid any oil contamination.

21. INCREASED MOISTURE IN THE WEB

22. Increased Moisture • Sudden reduction in touch roll load, vacuum or some other process variable may result in increased moisture in paper web. Due to increased moisture, when a hot yankee comes in contact with plenty of moisture, its surface temperature reduces significantly. This thermal shock can also affect yankee adversely.

23. Increased Moisture • To avoid such situations, system must be audited for its safeguards against failures. • In a typical case, it was noticed that the touch roll load reduced to such a low value that it lowered resulting in paper web break at yankee section, and chocked uhle boxes. Only a timely emergency stoppage could protect felts from permanent damage.

24. Increased Moisture • For the case above, the mill installed a pressure switch in pneumatic pipeline to activate an alarm if the pneumatic pressure reduced below minimum desired pressure. Compressor failure, severe leakage in pneumatic piping etc. result in alarm so that the machine operator can take a corrective or preventive action as the case may be.

25. PROTECTION TECHNIQUES

26. Pressure Inside Yankee

27. PRESSURE INSIDE YANKEE • An inlet pipeline is given on the following slide to illustrate how pressure measurement and control can be effectively done for a yankee.

28. Typical Yankee Inlet Piping Schematic PS PI SV To Yankee From Steam Header CV CV Hooter PS: Pressure Switch PI: Pressure Gauge SV: Safety Valve CV: Control Valve

29. Yankee Inlet Piping • It is very important to understand that the pressure indicated by a pressure gauge -may it be a bourdon tube type, or a pressure transmitter with indicator or controller- is NOT the pressure inside yankee as it is considered. It IS the pressure at inlet of rotary joint, and the pressure inside yankee would be gauge pressure minus pressure drop across rotary joint.

30. Yankee Inlet Piping • As the steam consumption, and hence steam flow rate increases through yankee, this pressure drop across rotary joint increases. That means, you actually provide much lower pressure inside yankee than the pressure gauge indicates. • Though seem unbelievable, this pressure drop may be to the tune of half of the gauge reading in some of the cases, i.e., if the gauge indicates an inlet pressure of 3.2 Kg/cm2, fair chances are that the inlet pressure id 1.8-2.0 Kg/cm2 only.

31. Measuring Actual Yankee Inlet Pressure • Well, it is not practically possible, and advisable too, to install a pressure gauge on rotating yankee. But the same can be done using a trick. • Condensate is removed form a cylinder at a temperature corresponding to the saturation temperature of steam at that pressure. • If a temperature gauge is installed just after rotary joint, we can find out the steam pressure inside yankee.

32. Measuring Actual Yankee Pressure • The results obtained by this technique are often surprising. In some cases, it has been found that the hydraulic testing of yankee at the fabricator site had been done at a pressure 8 or 10 times more than that is required for papermaking. • But, does that mean yankee is safe?

33. Yankee Pressure- Operational Vs. Intermittent • In most of the cases, when the paper is not running on machine, heat losses through yankee shall are low, and hence steam flow rate. That is why, the pressure drop through rotary joint reduces and pressure inside yankee increase, even though the pressure gauge indicates a reduction in the same.

34. Yankee Pressure: A Typical Case • In a particular case, a fabricated yankee was analyzed as discussed above. The design and operating data are as under- • Diameter: 4.2M • Shall Thickness: 62mm • Material: A36 (ASTM) • Pressure at Inlet Piping: 3.3-3.7Kg/cm2

35. Yankee Pressure: A Typical Case • That mill earlier considered that the pressure inside yankee is to the tune of 3.3-3.7Kg/cm2. But, considering the above, temperature measurement of the condensate just after rotary joint was done, and found that the condensate temperature was to the tune of 127-128°C. This corresponds to 2.0Kg/cm2.

36. Why to Install Safety Valve? • In case of paper running over yankee, it extracts heat from yankee, as a result of which, pressure reduces. But when the paper is not there, e.g., in case of a paper break, the pressure inside yankee may increase and affect the yankee adversely. To avoid the same, a safety valve or pressure switch connected to an alarm is recommended.

37. Non-Condensable Gases • Non condensable gases are another major source of pressure increase in yankee. • A minute quantity of non condensable gases can easily reduce down heat transfer rates significantly, due to which the operator is bound to increase steam pressure to maintain the desired production rates.

38. From Where Non-Condensable Gases Appear? • In case where boiler feed water is hard, and boiler water is also highly contaminated, some de-scaling chemicals generate gases like CO2 etc. • Also, if boiler feedwater temperature is very low, presence of dissolved oxygen and air in feedwater is released within boiler and these gases travel alongwith the steam to enter yankee. From the yankee, these cannot be easily removed as steam trap does not normally allow these gases to escape out.

39. Detection of Non-Condensable Gases in the System • Often it is very difficult to detect presence of these gases within the system, particularly if there is a scope of increasing production rates. • On machines where the production is limited by drying capacity, an increase in pressure could be an indication for the same.

40. Detection Techniques • For every grade of paper, and for every speed and basis weight, note down the normal operating pressure. • In absence of any other system, any increase in pressure could be a possible indication of presence of non-condensable gases in the system.

41. Detection Techniques • Here, the properties of steam can be used to detect the presence of non-condensable in steam. • We know that Boiling point of water increases with increase in pressure. • Hence, the temperature of condensate just after the rotary joint increases with increase in pressure inside yankee. • Due to presence of non-condensable gases, heat transfer is reduces, which is compensated by increasing steam pressure inside yankee.

42. Using Condensate Temperature… • Thus, regular monitoring can reveal presence of non-condensable gases in yankee. • For example, if normally the condensate temperature id to the tune of 125-127°C, any increase beyond 5°C can be considered possibly due to the same.

43. Minimizing Non-Condensable Gases • The first step is to minimize the generation of these from the boiler itself. • Desired number of blowdown, use of suitable descaling and scale preventing chemicals, timely descaling, maintaining appropriate feedwater temperature should be observed.

44. Minimizing Non-Condensable Gases • In case non-condensable gases have entered the system, another simple technique is to let it blow away. This can be done by bypassing steam trap in the condensate line. Though this may lead to minor steam leakage, yet in many cases, the effect is noticeable, and condensate temperature decreases indicating clearly that the heat transfer has improved substantially.

45. INCREASED CONDENSATE LEVEL

46. Increased Condensate Level • The problem with increased condensate level is that in most of the cases, it go unnoticed for a long time. It is quite possible that machine is running smooth without a break of paper for a long time, and sudden effect of condensate level increase is noticed.

47. Increased Condensate Level • In a typical case, vibrations started to appear in yankee framings, and it was decided to shut the machine and check yankee internally. Boiler steam supply was discontinued, and it was decided to run the paper without steam as long as possible to get yankee cooled down early.

48. Increased Condensate Level • Under that situation, machine could be run for nearly 30-40 minutes without steam. • On checking, it was found that the yankee was ¾ full (height wise). • One can understand how dangerous it can be to operate a yankee under such circumstances, but without knowing the condensate level in yankee what can be done?

49. Condensate Removal per Rotation of Yankee • It is easy to estimate condensate removed from yankee per revolution. Divide machine speed with yankee circumference to achieve yankee RPM. Now divide condensate removal (or steam consumption) per hour and divide the same with 60 (to convert hours to minutes) and then with yankee RPM.

50. Condensate Removal per Rotation of Yankee • For a typical case, steam consumption was 3000 kg/Hr. Machine speed was 210mpm and yankee circumference was 13.3M. • So the condensate removal per yankee rotation would be calculated as- • Yankee RPM = 210/13.3 =15.79 rpm • Condensate = 3000/(60*15.79) = 3.16 kg per revolution.