Primo Fireworks became interested in High Density Polyethylene Pipe as a possible replacement for spiral and convolute wound paper mortars and steel pipe after hearing several field reports regarding the use of that material in the industry.
Our interest was initially less an intrigue with HPDE directly as much as a dissatisfaction with paper and steel pipe as the method of choice for fireworks mortars. However, it turns out that our initial experiments with HPDE suggest that it directly solves most of the major problems that we have been having with paper and steel systems.
It will be helpful to explain our concerns with paper and steel systems in order to interpret and put in perspective our tests results and to understand why we have tested HPDE in the way we have.
Paper tubes represent a two edged sword as far as a material for fireworks mortars. On one hand they appear to have less of a fragmentation hazard than metallic mortars, and thereby would seem to represent a safer system than steel, however because of some unique properties of paper their relative safety is not as clear cut as it might seem at first look.
Paper tubes have very poor storage and use characteristics. For simple economic reasons it is not feasible to use paper tubes only one time and then discard them. Because of reuse over a period of several shows over a short period of a few weeks, or a longer use cycle (which is more typical) of a few shows over two to three years, paper tubes exhibit the following defects:
Typically a shell can be placed in a mortar using the industry accepted rule of thumb that if the shell falls to the bottom of the mortar under its own weight the shell can be considered 'safe to fire' from that mortar, all other factors sperate from specific considerations of the mortar but affecting show safety being taken care of and 'safe'. Just specifically talking here about the physical system of the shell as a projectile, and the tube as the launching system, we usually consider that system to be 'safe' and functional after applying our 'rule of thumb'. Unfortunately, paper mortars with bad 'delta' and unwinding leaves will cause a shell to 'lock' when discharged. This is a result of the great disturbance of the gas pressure and the 'chatter' of shell against the side of the out of round mortar as well as possible contact with paper leaves above the head of the shell. Typically the shell is over pressured at this point sometimes resulting in loss of structural integrity of the case wall, and subsequent fire transfer to the contents of the shell.
It is worth noting that 'unwind' can not be reliably detected in all paper tubes at all times in all cases. Usually the operator will inspect tubes for gross unwind and remove offending inner leaves that have become visibly loose. However, there is no reliable way to guard against a paper leaf that will become loose at the moment of discharge in response to the turbulent gas jets and great physical shock imparted by the lifting charge.
Additionally, canister shells with the now popular molded styrene lids are more prone than ball shells to 'shear' pieces of paper off the walls in the head-space area of the shell with resulting problems of over-pressure. We have specifically had bad experiences with pressed comets fired from paper mortars that exhibit this 'shearing' of paper the inner leaves of the paper mortar. The most extreme example of this problem is explosion of the paper mortar as the comet 'locks' against the walls of the mortar. We have been able to repeatedly reproduce this phenomena to our satisfaction that the cause is abrasion of the paper wall and subsequent over pressuring of the tube. It is our opinion that plastic molded end cap shells with non-radiused designs fired from paper mortars introduce a statistically significant increase in failure rate of these shells. Particularly troublesome are salute shells using this design.
There is no economically effective way to solve problems of paper tube unwind and degradation of tube shape over time as a result of enviormental factors. Short of temperature and humidity controlled storage, which is financially unacceptable, and therefore a non-solution, it would seem that what ever increase in failure that such factors cause in paper tube systems is irreducible and a fact of life when using paper mortars.
A further, specialized concern, regarding papers poor qualities in response to moisture occur in large electrically fired display programs. There is a growing segment of the display business that centers on large displays (over 1500 shells, excluding finales). The typical method employed on such displays involves setting paper tubes in sand filled trough boxes in long rows, typically 60 feet in length and 2 feet wide. Rarely is dry sand available, and because of the sheer size of these programs, display site setups are typically begun 2 days to 5 days previous to the day of the display. Paper tubes rapidly absorb moisture from the sand and swell. During subsequent drying out of the mortar for storage, papers problems of elasticity surface and greatly reduce the useful life of the mortar. If conditions are extreme at the firing site i.e. high humidity, rain, snow or ice melt and refreeze,or other poor weather and enviormental conditions, papers ability to readily absorb moisture may lead to an outright loss of wall strength sufficient to cause rupture upon discharge of the lifting charge and subsequent low break of the firework shell. Attempts to guard against this are marginal in nature. We have previously employed various 'shrink wrap' plastics to wrap paper mortars, but consider the practice only partially successful and in no way a solution if local environmental conditions deteriorate to heavy rain or very high relative humidity conditions for longer than 24 hours.
The excellent flammability characteristics of paper tubes is an often over looked safety hazard. Although it is almost a universally observed rule in the industry that paper tubes will not be reloaded when hot, papers ability to readily support combustion has insidious ramifications. There have been several documented truck fires caused by glowing embers inside of paper tubes, gone unnoticed by the crew, and returned to the bed of a transport truck and driven off down the highway. The increased airflow from the relative wind of the vehicle has provided the needed oxygen. I know of one such fire that occurred on a truck with other shells on board that were destined for another display. This ia a serious and real concern, that must not be overlooked. Our crews are specifically instructed to allow minimum cooling times for paper racks, and we have adopted further procedures for transport of multiple shows on a single truck with paper racks.
Papers poor elastic qualities in response to mechanical stress need no elaboration. Driving over a paper tube certainly renders it a useless deformed item destined only for the refuse can, but also the normal activity of handling the tubes on a display site impose stresses that soon render the tube in visibly poor condition. In fact rapid and continual degradation of the tube during normal handling is both exasperating to the display operator and a source of economic concern. It might be fairly stated that out of a given sample of any 1000 tubes in use by a display company, there is a fairly typical breakdown of tubes of relative quality that might be approximately as follows:
Steel has but one significant benefit. It is very strong, and able to withstand great pressures.
But its one drawback is so overriding in its nature that it renders steel as a fireworks mortar material totally unacceptable. That drawback is the extremely hazardous failure mode of steel when over stressed. Steel pipes become dangerous bombs transferring the absorbed energy of the pyrotechnic compositions into life threatening projectiles with enormous retained energy. Additionally these projectiles have all the worst characteristics of such objects. They have very high mass, they have high hardness, and they have razor sharp edges.
There has been some discussion of various types of steels, and there has been some discussion of what is wishfully referred to as 'non fragmenting' metals, but the only intelligent comment I have heard regarding the use of steel pipe came from Shimizu. He states that steel mortars should be made of welded cold rolled steel, and the weld seam be placed away from the operator. Your only hope with an over-stressed steel pipe is that the weld will totally fail in response to over-pressure allowing the tube to 'unwrap'. Considerable energy is required to unwrap the steel pipe with an attendant transfer of that energy into heat which is absorbed by the steel plate. The weak weld point serves the useful function of a 'safety valve' which breaks before the steel tube, with its more uniform strength characteristics, shatters into the above mentioned projectiles. The use of so called 'seamless' tubes is specifically admonished against by Shimizu for obvious reasons.
Of course steel has other undesirable characteristics, which are more of annoyances than problems. It is heavy, and possesses problems of transportation and handling. Working with steel is a notoriously dangerous thing to do. 10" steel pipes in lengths necessary to use as fireworks mortars can easily weigh enough to remove the foot or arm of a careless operator who allows such a pipe to accidentally roll off the back of a truck bed and onto the offending human appendage. There have been so many documented accidents with steel pipes that it seems unnecessary to list them here. I have personally witnessed a 6" steel pipe tossed 275 yards in an arc several hundred feet high when the mine it was supposed to shoot detonated. The released energy of that steel pipe as it contacted the earth would be interesting to calculate. Sufficient to say that you would most likely not volunteer to catch it even if you could see it coming. Last year in California a man was injured as he stood almost 100 yards away from a steel mortar that fragmented. The reach of hard steel as it flies through the air is nothing to be taken lightly.
I have heard the following comment stated by many people recently. "The fatal and serious fireworks accidents could be reduced significantly if we could just get rid of steel mortars." I agree one hundred percent.
For our tests we obtained a sample of HDPE pipe from a West Coast manufacturer of polyethylene pipe. The product is manufactured under the trade name Duratuff. Specifications are as follows:
Nominal Mortar Size Length I.D. Wall O.D. SDR# Weight ------|----------|------|------|------|------|------- 3" 18" 2.97 .259 3.50 13.5 1' 10oz 4" 24" 3.97 .265 4.50 17.0 3' 1 oz 5" 30" 5.02 .265 5.56 21.0 4' 11oz 6" 36" 5.99 .316 6.63 21.0 7' 9 oz
The resin used in the pipe conforms to ASTM D-1248 Tyype III C Class C, Category 5, Grade P34. The resin is PE3408. Finished pipe is made to ASTM D-1248 specification. This pipe contains 2.5 percent by weight Carbon Black as a U.V. inhibitor. It is worth noting that some polyethelye pipe is not U.V. inhibited and it should be avoided as a candidate for fireworks applications. Most typically this is medium density pipe which is bright orange in color and is commonly used as buried gas main.
Mortars were plugged with wooden disks turned from Southern White Pine that are 1.5 inches thick and turned to approximately 1/32" oversize for an interference fit into the tube. Plugs for mortars larger than 3" were made by laminating multiple disks with the grain of the wood turned to run at right angles each other. The following plugs were used:
Certain known characteristics of HPDE allow us to draw some conclusions without ever submitting the material to test. Firstly one of our concerns deals with paper inner leave degradation. Since HPDE has no such inner leaves we can assume that this problem is not a factor with HPDE. However, abrasion of shells and other pyrotechnic objects, i.e. comets, against mortar walls is a significant factor. A feature of fireworks is that limited testing of individual events will produce only a finite amount of knowledge about the subject as it relates to intensive field use of an item. This results from the fact that fireworks operations involve numerically large numbers of individual events.
As an example, a typical display company may fire 80,000 to 160,000 three inch shells in a three month period around the fourth of July. Typical acceptable failure rates are in the range of one tenth of one percent. It is very difficult to impossible to design a laboratory testing procedure that will provide any meaningful information about such an inconveniently small statistical sample within that large field. At some point in time, one must move to actual field testing to gain the required data. Additionally, any models of the great diversity of display fireworks field situations that may be fabricated in a laboratory environment would be questionable on their face.
The abrasion of comets and shells against the walls of a HDPE mortar is such a problem. Only extensive field experience will tell us whether or not HPDE is beneficial in this regard. It should be noted that HDPE has excellent abrasion resistance, and excellent gloss. It would seem likely that problems of 'locking' of pyrotechnic devices in HDPE tube would be minimal.
Our second concern when discussing paper mortars pertained to flammability. HDPE was tested by focusing the flame from a common propane torch on the end section of a 4" HDPE mortar. We were not able to create a set of circumstances under which the yellow flame present when the torch was held in a steady position on one section of the plastic for over 30 seconds would sustain itself after the propane flame was removed. We did cause the plastic to fully melt and run in a liquid down the side of the pipe section. We do not believe that poor flammability characteristics will be a significant problem with HDPE.
Our third concern was deformation through handling. Elasticity of HDPE is excellent. We attempted to deform typical mortars by crushing their full lengths with various weights. We removed the end plugs for this test as the plugs would provide additional strength to the mortar over a certain amount of its length. The test configuration is comparable to the stresses that a typical mortar might receive in normal handling when placed under other display equipment in a transport truck, or during storage. The results are as follows:
Time
Weight
Size Length Weight Applied Maximum Deformation Recovery
-----|----------|--------|---------|---------------------|-----------
3" 18" 150lbs 24hrs 0 n/a
3" 18" 200lbs 24hrs 0 n/a
4" 24" 150lbs 24hrs 0.125 100 %
4" 24" 200lbs 24hrs 0.375 90 %
5" 30" 150lbs 24hrs 0 n/a
5" 30" 200lbs 24hrs 0.375 90 %
6" 36" 150lbs 24hrs 0.125 100 %
6" 36" 200lbs 24hrs 0.375 90 %
---------------------------------------------------------------------
Our fourth and fifth concerns relate to moisture. Of course HDPE is non soluble in water, and has no wicking or water absorbtion characteristics.
Continual expansion and contraction with moisture and temperature is a major concern with paper. HDPE has a thermal expansion rate of 1 x 10-4 inches per degree F. This rate can be considered insignificant for our purposes. It has been noted by Syd Howard that wood plugs in HDPE mortars last significantly longer than equivalent paper mortars. I suspect HDPEs' low thermal expansion factor is the reason why. Since HDPE does not retain or absorb moisture, expansion or 'swelling' as a result is a non-issue with this material.
Our fifth concern is with the 'delta' or out of roundness from the design specification. This is where we have encountered our first problem with HDPE. The sample lot of pipe that we have had significant and unacceptable delta factor. Out of round deviation as great 0.0500 was common in the sample. Our design requirements are for 0.0250 maximum delta. We have been assured by the manufacturer that this problem can be taken care of. Our statement to the manufacturer was that pipe with this delta would not be accepted and that this is a condition of sale. We received no hesitation on their part in agreeing to the design specification.
In the general discussion of paper mortars several items were mentioned including special concerns at large electrical displays and unacceptable 'cycling through' of largely varying quality paper mortars in typical display operations. It is noted that HDPE is water proof and also that it may be conveniently capped with commercially available plugs ( Caplugs Corp ) to render the mortar assembly a water tight sealed unit eminently suitable for such electrical displays. The material is resistant to brittleness at temperatures to -180 F. Show operation in sub zero conditions should prove to be no problem.
It is further suspected that actual field use will prove out what we suspect to be true of HDPE mortars in general as regards the 'cycling through' of mortars. Because of the unique combination of elasticity, abrasion resistance, thermal stability, and water and moisture resistance of HDPE a typical fireworks operation should see a 'flattening out' of the curve of good to excellent quality mortars in stock over a period of years with a significant increase in the numbers of good to excellent mortars as paper mortars are phased out of the operation. HDPE mortars that are no longer acceptable for use will likely be those that have suffered catastrophic injury from malfunctioning shells, rather than those suffering from continual degradation in performance level.
Turning now to an examination of HDPE performance when compared to known defects in steel pipe we will consider first the most major defect of steel pipe performance in fireworks operations. That is the failure mode of steel which is to fragment when over stressed. We note that HDPE pipe has a tensile yield strength of 3500 PSI which makes it a very strong material indeed, but well short of steel, and that it has an elongation at break of 800 % compared to steel which is almost nil depending on the exact alloy, its age and temperature. We might surmise from that information the approximate behavior of HDPE under the extreme and rapid over pressure that would occur in a typical fireworks shell detonation. We devised a series of field destructive tests to see if, in fact, HPDE would behave as we guessed it might.
In all tests we placed the mortar upright on flat level ground in a free standing position with no supports. Shells were ignited using an electric match. The tests were recorded on video tape from two angles of view. One was a close up of the mortar, and one a wide angle view taking in the area of the mortar and approximately a 50 yard area around the mortar. Subsequent to the tests the video was transferred to a digital slo-mo machine for review.
Our first test was with 3" mortars. A typical U.S. made single break color shell was modified by removing the delay timer so that it would explode with ignition of the lift charge. No other aspect of the shell was changed. Lift charge was left on the shell. The mortar suffered no damage from this test. The contents of the shell shot out of the end of the mortar in a manner that might be described as a very high performance mine effect.
The second test was the same as the first except the shell type was a Japanese manufactured Chrysanthemum. The shell was similarly modified by removing the timer, and the lift powder was also removed so that the shell would remain at the base of the mortar. There was no damage to the mortar from this shell.
The fourth test was a domestic 3" salute shell with the timer removed. The salute shell contained approximately 2 ozs of aluminum/perchlorate flash mixture. The mortar was destroyed by this shell. We recovered a total of 11 pieces of the mortar scattered in a circular pattern in a 35 foot radius about the original position of the mortar. The pieces were in three distinct weight and size groups. One large piece was found with a weight of 1 lb 1.25oz which accounts for 68 % of the original weight of the mortar; One piece was 3 ozs which accounts for 12 % ; Additionally 9 very small pieces were found whose total combined weight was 1.75 ozs. The small pieces had been stretched very thin, nominally 0.08 - 0.10" and were uniformly very flexible to the touch and feathery in appearance with many little 'fingers' extending out from the perimeter. Characteristic of all our tests of HDPE, these pieces had no sharp edges what-so-ever. The edges of the larger pieces also showed this characteristic 'feathering' of the edge surfaces where the plastic appears to stretch into 'strings' at many points before finally separating from the parent material.
The fifth test was a modified 4" Japanese Chrysanthemum with lifting charge but no delay timer. There was no visible damage to the tube.
The sixth test was a modified 4" Japanese Chrysanthemum with no lifting charge and no delay timer. The tube exhibited a slight and permanent bulging of its lower portion just above the top of the wooden base plug. The plug was pushed out of the end of the tube so that about half its length was exposed. However the nails were still firmly attached to the plugs but bent in a severe 'S' shape. The wall of the tube was able to resist what must have been tremendous shearing force that was capable of bending the nails, yet retaining the nail heads in position with very little deformation of the tube material around the nail head. I found this quite remarkable and rather unexpected. A comparable paper tube would have been ripped to shreds in the nail head area under comparable stress.
The seventh test was a modified 5" Japanese Chrysanthemum with no lift and no delay timer. The tube ripped open at its base, again exhibiting the characteristic feathering and fingering at the separations. The tears at the base of the tube appeared to originate at the location of the nail and proceed vertically up the walls of the tube. It is likely that the nail holes introduced a weak point in the wall of the tube from which a tearing action moved directly up the tube to about a third of the tubes length. The total weight of the tube was retained with no pieces separating from the body of the tube.
The eighth test was a 6" modified Japanese Chrysanthemum with lift charge and no delay timer. The base plug of the mortar blew out of the tube as the tube opened up with the same vertical shears as appeared on the seventh test, and it appears that the shell traveled about half way up the tube before exploding. At that point the middle of the tube opened up in splined fashion with about five equally spaced fissures about the girth of the tube extending one foot from the mid point of the tube in either direction. Again the edges of the tears exhibited the characteristic feathering. The total weight of the tube was retained with no loss of material from the main body of the tube. Upon explosion of the shell and the blowing clear of the base plug the tube was tossed straight up in the air as if lifted from the bottom by a rocket. The tube traveled about 20 feet in the air, took a slight trajectory to one side and them began to tumble rapidly. At that point it began to arc back toward the ground and landed about 36 feet from its original location.
We observed what would normally be a worst case tossing or throwing of a mortar during shell malfunction with HDPE. The six inch sample was separated from its plug in a manner that tossed it into the air with most of the energy directed to the plug end. It is our opinion that if this were a high density steel pipe it would have been thrown several hundred times farther than the HPDE sample. Also it must be noted that the HPDE sample pipe in this case is several orders of magnitude lighter than an equivalent steel pipe, and therefore carries with it substantially less energy than steel as it lands. We feel that with normal on-site safety clothing i.e., heavy cotton or wool coats, head protection, safety glasses, leather gloves, contact with such a tossed mortar would be unpleasant but in all but freak cases survivable, and most likely innocuous in the vast majority of cases.
HDPE seems to fail in a characteristic manner, by ripping vertically along its length. Beyond that the torn edges of the material exhibit feathery, soft edges that are not sharp in nature and that are not capable of cutting flesh. The particular fashion in which HDPE fails and the characteristic nature of the edges can be partly explained by polyethylenes thermoplasticity which is its ability to be deformed without breaking with a relatively fast flow, when heated to a suitable temperature and properly stressed. We believe that elongation at break is only part of the explanation for the failure mode of HDPE. Under the extreme shock and pressure of shell detonations a certain percentage of the available energy is used to heat the stressed areas high enough to enhance the thermoplasticity of the material. This in combination with HDPEs excellent elongation at break explain its predictable and desirable failure mode.
It apparently takes high velocity shock waves, such as found in salutes, to cause HDPE to separate into smaller pieces. It is our opinion that given, the nature if these pieces, it is quite unlikely that they represent anywhere near the hazard rating that we would ascribe to equivalent steel pieces. We would again suspect that with proper on-site safety clothing, the hazard of small flying pieces of HDPE, most of which weigh according to our tests well under one ounce each, with most averaging around 0.25 oz, could be reduced to insignificant levels. Injuries to unprotected flesh would be most likely bruises. It is difficult to imagine these pieces penetrating flesh.
Conventional steel manual operation show 'reload' type operations that are still in common use through out the industry may be able to be replaced with an HDPE version of that system. Sand filled trough boxes have been used with steel pipes for this type of operation. The main purpose of the sand has been to provide a dense medium that would slow the travel and absorb the energy of steel pieces that would be created in a failure of the pipe. Given enough sand, proper installation of the pipes in the box, and a failure in the lower two thirds of the trough box, this techniques works with some success, although it can not be considered one hundred percent fail-proof.
We believe, from observing the splitting open, and 'venting' characteristics of HDPE that a system could be devised to re-direct the gas vented by a failing HDPE mortar away from the manual show operator. Additionally this system could protect the operator from whatever minor debris might be created by the failing tube. We think that such a system could have substantial benefit over steel box and sand filled systems. Among these would be lower cost, faster on site assembly, much greater a likely percentage of fool proof safety under failure, and because the assemblies are a permanent arrangement that is carried to the firing site, the variance in final construction would not occur as it does in trough boxes filled 'on-site' by the operator and crew.
There are several techniques in use in the oil and gas industry to weld HDPE pipe. We are looking at these to determine if a replacement for the wooden plug can be used with HDPE. Unfortunately, the wooden plug may compromise whatever safety benefit is gained by HDPE under a certain small percentage of failures. Debris from the wooden plug and nails is something that we should work toward eliminating. An integral plastic plug assembly 'welded' into the base may prove to be the answer.
The future for HDPE looks bright. It has a unique combination of benefits that should be of substantial interest to the fireworks industry, and we suggest you look at HDPE to see if it may be of use to you. We are continuing to experiment with this material and will give you our findings in future reports. We are interested in hearing from other operators regarding their experiences with HPDE. You may write us at P.O. Box 357 Madera, California 93639.