A water management system that cools and re-circulates the water can reduce this water consumption to just a few gallons per year. This water management system also increases the pump reliability and mean time between failures (MTBF) significantly, with a return on investment (ROI) that is usually around 6-12 months. Additionally, significant energy savings is documented through the use of this system, by greatly reducing or eliminating the amount of energy needed to heat the flush water up to process temperatures, and then to boil/evaporate this water from the product.
Water, Water Everywhere…
“Almost 3 billion people will face severe shortages of water by 2025 if the world keeps consuming water at current rates…” United Nations Report
“Even where supplies are sufficient or plentiful, they are increasingly at risk from pollution and rising demand. Fierce national competition over water resources has prompted fears that water issues contain the seeds of violent conflict…” Kofi Annan, UN Secretary General
“The simple fact is that there is a limited amount of water on the planet, and we cannot afford to be negligent in its use. We cannot keep treating it as if it will never run out…” Mohammed El Baradei, Head of the International Atomic Energy Agency
Of all the world’s water, 97.4% is salt water, 2% is frozen in glaciers and ice caps, and only 0.6% is available for human consumption and industrial use. As the above quotes illustrate, our water supplies are finite; the sources of supply are becoming increasingly polluted, and are asked to supply ever-larger volumes. The United States is struggling with long-term water shortages in California and the Southeast, shortages which will not be overcome by one or two good years of rainfall. We have also heard about the serious drop in the water level of the Ogallala Aquifer which underlies 8 central states, from North Dakota to Texas, due to over-pumping of this water resource for crop irrigation.
Clean, fresh water is a limited resource which is under increasing pressure on our planet. There are compelling reasons to actively pursue water conservation measures that are both proven and economically justified.
The Impact of Industry
The 2nd most common machine in industry (after the electric motor) is the pump (Figure 1). It is estimated that there are about 600 million industrial pumps in the world (not counting about one billion pumps in domestic use, such as dishwashers, clothes washers, automobile water pumps, etc). These industrial pumps rely on a mechanical seal (Figure 2) or packing to seal the rotating shaft and contain the pressurized liquid within. Mechanical seals and packing require clean fluid, usually water, and lots of it, for cooling and lubrication. As our world has become more industrialized, the population of pumps has grown accordingly, and so has our demand for clean water to service these pumps.
However, the rising cost of water, along with increasing awareness of the long-term consequences of unrestricted water use, has caused many forward-thinking companies to reconsider their traditional, wasteful water use practices. Moderate investments in proven new water-conserving technology can achieve financial payback within the first year, while also having beneficial effects on our global water supply that will last for decades.
The Importance of a Fluid Film
Process pumps usually usea mechanical seal to contain the pressurized fluid by creating a sliding seal between the rotating shaft and the pump housing. This mechanical seal is engineered to operate with a thin fluid film separating the highly-polished rotating and stationary seal faces (Figure 3). The face materials and seal design are selected specifically for the particular pump application parameters and fluid properties (pressure, temperature, viscosity, etc.). The fluid film may consist of either the pumped process fluid, or a special fluid such as clean water may be introduced under pressure if the process fluid is not suitable (e.g., if it is too hot, too high solids content, tends to crystallize, etc.)
If this fluid film is not stable or not present, the two faces will contact, overheat, and damage each other and the mechanical seal will fail, causing the pump to fail (see Figure 4). When the seal fails, the entire pump unit is removed and repaired, at an average cost of $2,500 per repair. This is the maintenance cost (parts and labor) only and does not include the value of lost production, which can be thousands of dollars per hour.
The fluid film can be adversely affected by process upsets that lead to dry-running of the seal, which can include:
• Process changes upstream that lead to no liquid product at the pump.
• Operator error (opening or closing a valve which stops product flow to the pump, etc.)
• Cavitation, where there is inadequate net-positive suction head to the pump and the liquid product changes into a vapor state at the impeller.
Mechanical seal faces can also be damaged by product crystallization due to temperature and pressure changes across the face. Suspended solids in the pumped liquid are another major cause of seal damage, and these solids must be kept away from the faces with a supply of clean water to form the fluid film or the seal will fail prematurely.
Old “Water-to-Drain” Plan
Many industrial plants supply their double mechanical seals with a fresh supply of barrier water using the “water-to-drain” method, or API Plan 54 (shown in Figure 5).
In the water-to-drain plan, plant water is supplied to all of the double mechanical seals in parallel from a water header main. This plan can work satisfactorily if all conditions start out perfectly and stay perfect, but as we know, this level of perfection is rarely found or maintained in a real, operating, industrial plant. The water-to-drain plan is prone to process upsets that cause seals and pumps to fail, for example:
1. Water will take the path of least resistance. If the pressure and flow rates of each branch of the piping are not set exactly right at startup, most of the water will selectively flow through the pipe with the least resistance, leaving the other seals to starve for water and seal failures can result.
2. If one mechanical seal fails and allows process fluid to enter the water supply line. All of the seals can be cross-contaminated and thus lead to multiple seal failures.
3. If an alternative flow path is created, for example by an operator turning on a large water line for wash-down, the pressure and flow of water to the seals will drop and this could lead to seal failures.
4. If the quality of the water being supplied by this system is poor, it will lead to solids being delivered to, and collecting on, the seal faces; this will lead to premature seal failure.
In addition to these failure modes, water-to-drain has the following cost considerations:
• If the supply water is being purchased from a municipal supply, the cost of the water can be very high, as can be the cost of treatment and disposal of the waste water.
• The typical water flow rate on a water-to-drain plan runs an average of 3.2 gallons per minute to drain. Running 24/7, this amounts to a staggering 1.7 million gallons of water per year, per pump, running to the drain.
New Water Saving Solution
The solution to the shortcomings of the water-to-drain plan is to install a water management system tank above each pump (shown in Figure 6). Recall that the sliding faces of the mechanical seal (at lower left in Figure 6) create frictional heat in the seal. Heat is also added to the seal by the hot pumpage. Hot barrier water from the double seal rises up to the tank via the upper tube, where the heat is radiated to the atmosphere; the cooled barrier water is then returned to the seal through the lower tube. Circulation of the barrier water from the seal to the tank, and back to the seal, is maintained by the thermo-syphon effect (basically, hot water rises and cool water falls), with no moving parts. In cases where more flow of the barrier fluid is required, pumping assistance can be obtained from an optional bi-directional pumping ring in the seal itself.
The tank is connected to the plant water main and automatically tops up with water to replace the very small amount (about 30 gallons per year) of barrier water that is lost at the seal faces during normal operation. The tank is maintained at a pre-set pressure that is one bar (15 psi) above the pump’s stuffing-box pressure, to maintain a positive pressure differential across the seal faces that ensures clean barrier water is forming the fluid film (as opposed to process liquid). Additional cooling can be accomplished, where necessary, by adding fins to the tubing, adding a cooling coil to the tank, and/or using a larger tank.
A diagram of a typical multi-pump layout is shown in Figure 7. Each tank system serves just one seal/pump set, and is isolated from all other tanks. (Note that only 3 tanks/pumps are shown in the figure, but many hundreds of tanks/pumps are often connected in practice.)
The advantages of this system include:
• The waste water-to-drain is completely eliminated, with huge savings in water resources, the cost of water, and the cost of treating waste water. A single tank/seal combination typically uses only about 30 gallons of water per year, thus typically saving 1.7 million gallons per pump per year.
• Water from the plant main line passes through a check valve which prevents contamination caused by a seal failure from passing back into the main, so each pump and seal is isolated; one seal failure does not adversely affect other pumps.
• A pressure regulator on the water feed line into each tank sets the tank pressure at the correct pressure for that pump, so each pump can operate at a different pressure and have a fluid film that is maintained at 1 bar (15 psi) over its stuffing box pressure. Note that the maximum pressure possible in each tank is equal to the plant’s main water line pressure.
• Each tank/seal/pump is a stand-alone system; changes in the operating conditions of one pump or the water flow to one seal do not affect the water flow to the seals of any other pump.
• Changes in the main line pressure, such as turning on a wash-down hose, do not affect the operation or the pressure in the tank systems, as each tank is isolated by its own check valve.
• The water quality is greatly improved by installing a filter on the incoming line to each tank. Since only about 30 gallons of water are used per tank per year, the filter will last a long time before plugging. Cleaner water leads to longer seal life.
In addition to eliminating water-to-drain and the pump seal life extension, there is one more way that water management systems save plants large sums of money, in those instances where the plant is using single mechanical seals. Figure 8 shows an API Plan 32 on a single mechanical seal, where clean water is used to flush and cool the seal faces. Cold water passes from the incoming line at upper left in the figure, into the stuffing box, and directly into the hot product. As much as 3 million gallons of water per year can be injected into the process liquid by a single pump using API Plan 32. In many processes this water must be heated up to the process temperature and ultimately evaporated out of the product, using large amounts of additional energy.
On a typical pulp and paper mill sealing application, with the incoming flush water at 60° F, and mixing with process fluid at 140° F, the cost of energy for heating the flush water to process temperature costs $4,000 per year. The energy to evaporate this water at the end of the process costs an additional $9,000 per year. Ironically, this API Plan 32 water addition is not easily visible and so this huge water usage, and resulting waste of energy, is often not even recognized as such by the plant operator.
A water management tank system used with a double mechanical seal to replace the single mechanical seal in Figure 8, eliminates both the wasted water and energy by re-circulating and cooling the barrier water to the seal faces.
CASE HISTORY #1: Tanks, Double Seal and Flow Fuse on Agitator
Guinness/Diageo, a major beer brewery in Dublin, Ireland, was experiencing failures of the single mechanical seals on their hot wort agitators about every 6 months. The vessels on which the agitators are mounted contain about 25,000 gallons each and must be drained completely for an overhaul, leading to a great deal of down time and lost production. When the seals failed, the wort leaked straight onto the floor, causing a loss of valuable product, as well as a safety hazard. Any proposed remedy had to also prevent the addition of water into the vessel, since dilution of the product was not allowed at this point in the process.
The proposed solution was a double mechanical seal with a water management system. Because the hot wort could not withstand dilution, the system includes the Flow Fuse automatic valve shown in Figures 9 and 10 below, which detects an abnormally high water flow rate and shuts off the plant water main supply to the tank system. This action protects the hot wort product by preventing water entry into the agitator in the event of an inboard mechanical seal failure.
The water management system package has been in place for 2 years with no seal failures (recall that the previous MTBF was six months). The payback period based only on the cost of repairs (not including lost production) was only 180 days.
CASE HISTORY #2: Tanks & Double Seals Replace API Plan 32 Single Seals
A pulp & paper mill in the USA uses API Plan 32 (Figure 8) to flush their single seals, with considerable energy required to (a) heat this flush water up to process temperatures, and (b) evaporate this excess water off at the end of the process. Five bleach filter pumps were fitted with water management tank systems and double seals in October 2008, and monitored over the following 9 months. First year savings (from energy, water, wastewater, and reliability) were calculated at $41,800, with an ROI payback on the investment cost (parts and labor) of the new tanks and seals of 1.2 years.
Spurred on by these excellent results, the paper mill has now completed a comprehensive mill survey to look for additional savings opportunities. A total of 604 pumps/seals were surveyed, and 144 pumps (24% of total) were deemed to be “highly energy inefficient”. A capital project is being considered to convert these 144 pumps to double seals and tanks. Annual savings are conservatively estimated to be $473,000 per year, with water savings of 174 million gallons per year.
CASE HISTORY #3 : Tanks Replace API Plan 54 Water-to-Drain
At a food processing operation with a total of 6,300 pumps, water management tank systems have been installed on 1,310 (20% of total) of the most arduous “bad actor” pumps over a six year period. Of the 1,310 upgrades, 750 (57%) are running with no failures since the upgrades began in 2001. Prior to the upgrades the seal life for the population of 1,310 pumps was 2.58 years. After the water management systems were installed, the seal life improved to 3.42 years.
The upgraded population of pumps had 2,127 failures prior to the upgrade, and only 1,059 failures after the upgrade (during the six year period). Using an average cost of a pump repair of $2,500, we find the following savings in avoided repair costs:
Cost of repairs prior to Upgrade: 2,127 failures X $2,500/repair = $5,317,500
Cost of repairs after Upgrade: 1,059 failures X $2,500/repair = $2,647,500
Repair Costs Avoided: $2,670,000
Total cost of the 1,310 water management systems: $1,345,000
The savings to the equipment owner, based on equipment life alone, resulting from the upgrade to water management systems is thus $2,670,000 - $1,345,000 = $1,325,000. Additional savings have been realized from the 1,310 pumps X 1.7 million gallons per pump per year = 2.2 Billion gallons of water saved per year.
Water management tank systems, used in conjunction with double mechanical seals on pumps, agitators, and mixers, will generate significant cost savings due to:
• Virtually no water usage and no associated costs for either buying city water or treatment of pond water. Savings of 1.7 million gallons of water per pump, per year have been frequently documented when used on API Plan 54.
• No waste water treatment and disposal; again, as much as 1.7 million gallons per pump per year on API Plan 54.
• No energy needed to heat the flush water introduced into the process up to process temperatures using an API Plan 32.
• No energy needed to boil/evaporate added flush water from the product (Plan 32).
• Increased pump lifetime and reduced pump overhaul costs.
• Less downtime and associated lost production.
These tank systems are pressurized by the plant water main to 1 bar (15 psi) above the pump stuffing box pressure. The tanks use no moving parts, simply relying on the “thermo-syphon” process to circulate hot water away from the mechanical seal, release the heat to atmosphere, and return cool water to the mechanical seal. This clean, re-circulated water extends the life of the mechanical seal and the uptime of the pump by flushing solids away from the seal faces, as well as cooling the seal faces. The systems are maintenance-friendly, requiring no external compressed air for pressurization, and do not require any manual intervention for refilling.
The standard flow indicator gives a quick visual indication of an inboard seal failure, which is difficult and costly to identify otherwise. An optional Flow Fuse will shut off the plant water supply when it detects an abnormally high flow of water, such as that caused by an inboard seal failure.
ROI payback periods for water management tank systems with double mechanical seals are typically 6 to 12 months, after which the tank systems continue to generate large savings for the remainder of their 10 to 20 year lifetimes. Chris Rehmann is Marketing Manager for AESSEAL, Inc., (www.aesseal.com), who design and manufacture mechanical seals, seal support systems, and bearing protection, and sell these products from offices in 32 countries. Chris earned a BSEE from the University of Notre Dame, and held various international management positions with an oilfield engineering firm for 15 years before joining AESSEAL’s North American headquarters in Knoxville, Tennessee in 2002. He can be reached at email@example.com.
All illustrations courtesy of AESSEAL, Inc., Rockford, TN, USA. Special thanks go to Cargill and Tim Goshert for their valuable contributions to this article, to Guinness/Diageo brewery for providing information and approval to use Case History #1, and to Terence McCarthy and Jonathan Broderick of AESSEAL, Ireland.