Clayton Armstrong 2011-02-22 GSK Aranda SP - steam&condensat

STEAM AND CONDENSATE AUDIT

Project N°30275

Date: 04/02/2011

GSK MANUFACTURING Aranda, Spain

Page 42 of 45

Established by E. Morin

To the attention of Ms. Beatriz Herrero-Gonzalo

controls will try to correct by closing the steam control valve. This cycle will repeat and control valves

will “hunt” searching for balance. Hunting control valves, and actuators, wear quicker and tend to

leak. The most critical aspect of cycling control valves is that the frequent changes in temperature

will cause local material stresses in the heat exchanger, which over time can cause failures and

leaks (especially in stainless steel). In addition the presence of relatively cold condensate may

cause water hammer and corrosion inside the heat exchanger which can also lead to leaks.

Lowering the condensate back pressure will reduce the risk of condensate backing up in the heat

exchanger, which provides two system improvements. First, it will reduce the loss of exchanger

capacity, and second, it reduces the risk of water hammer. Often when condensate is backing up,

the condensate lines are drained to the sewer. This is only a temporary fix and is a great loss of

energy and can raise waste water temperatures above safe limits.

On-off controls

As with modulating controls, very similar conditions occur in an on-off control. The steam valve

opens when there is a heat demand. A positive pressure differential is created, and the condensate

in the heat exchanger is pushed out. The heating surface in the heat exchanger is exposed and the

capacity rises. Before all of the condensate is pushed out, the desired temperature is reached and

the steam valve closes. During this cycle the steam trap does not receive condensate with a

temperature above 100ºC.

When the steam valve closes, the steam in the heat exchanger will condense, thus creating a

vacuum in the heat exchanger. This vacuum will pull condensate back from the condensate line

unless there is a check valve in place. The condensate inside the heat exchanger will continue to

cool down (sub-cool). When the steam valve opens again, the hot steam will be in contact with the

relatively cold condensate. When this occurs there is a serious risk for thermal water hammer to

occur. Over time these water hammers, and the presence of cold aggressive condensate, can cause

leaks.

Installing a vacuum breaker and a check valve may eliminate the vacuum and the backing-up of

condensate, but it will also allow air to enter the system. This air has to be vented from the heat