In a webinar available on the DFT Inc.’s website, Arie Bregman, vice president and general manager at the valve manufacturer’s Exton, PA site, presented background and guidelines for managing condensate in steam systems. This article is based on that webinar.
Many, if not most, industries use steam in some part of their operations for power generation, heating, chemical processing or other purposes.
Since steam is the vapor state of water, under various conditions (intended or not), the steam will condense and condensate water will be somewhere in the system. The condensate makes its way to the low points in the system, where unless removed, it’s likely to stay and cause damage to valves and piping.
Damage due to condensate may include:
Corrosion: Accumulated condensate water can pool in lines, valves and equipment. If allowed to remain pooled, the water can cause corrosion, even in corrosion-resistant materials. In addition, if carbon dioxide is present in the piping, the gas combines with the condensate water to form carbonic acid, which aggravates any corrosion problems.
Reverse Flow: Excessive condensate in the wrong place can result in condensate or steam flowing in the direction opposite to what’s intended. Condensate may flow back to where it heats up again and flashes back into steam. “Valves that have already suffered corrosion due to condensate pooling are particularly susceptible to reverse flow, which can lead to inaccurate flowmeter readings and potentially dangerous pressure spikes,” Bregman said in the webinar.
Steam Condensate Flashing: In areas where large quantities of condensate collect, the flow steam may push the condensate to where it flashes to steam. This produces tremendous forces that can destroy pipes, valves or heat exchangers.
PRESSURE SPIKES IN PIPING SYSTEMS
Three different pressure-spike scenarios can occur in piping systems, Bregman explained.
The first occurs in liquid-only systems; these pressure spikes can be caused by valves closing or pumps shutting down suddenly.
The second occurs in steam systems where condensate accumulates and then flashes to steam. The water, expanding to a vapor, increases in volume 500−600 times.
The third, and most dangerous, form of water hammer also happens in steam systems, and is commonly called condensate-induced water hammer. This happens when the pooled condensate is pushed by the high velocity steam traveling in the pipe, much as the wind pushes the water on a pond or a lake, creating waves. When the steam builds up a wave front in the pooled condensate, the flashing of the liquid from liquid to vapor can have dramatic, catastrophic consequences. The steam pushes the slug of water into an elbow or some other constriction at velocities in the hundreds of feet per second.
One of the more famous steam explosions happened in New York City in 2007 when a steam line under a street ruptured, blowing a hole in the pavement large enough to swallow a tow truck.
How can you prevent such damaging events from happening?
MANAGING CONDENSATE TO PREVENT DAMAGE
Careful design of the overall system and, in particular, the steam condensate system, can prevent most problems.
Properly sizing all the lines and valves in the system is of utmost importance. “Improperly sized pipes can be a leading cause of steam-condensate collection,” Bregman said. The location of condensate return lines in relation to other pieces of process equipment is also extremely important. Look for the low points in the system where condensate will accumulate.
Steam traps, devices that allow draining off of condensate without letting steam escape, should be installed at strategic locations to prevent condensate accumulation while retaining the steam in the system (Figure 2).
Proper insulation is also important in preventing flashing.
All lines and valves in the systems should be sized adequately and appropriately.
The type and quality of the valves in the condensate system should be considered, particularly check valves. Check valves can be useful in managing condensate, but off-the-shelf check valves may not do an adequate job of handling condensate; special modifications may be required, such as low-cracking-pressure springs. A soft seat material might be advisable for preventing erosion between the closure mechanism and the seat mechanism.
In addition to preventing condensate-related damage, effective condensate management allows capturing and recycling of condensate, minimizing the amount of water the system uses.